Normally I don't answer questions after the answered sign has been given, but I do believe that there is still much to say. So without much farther ado, let's dive in.
Questions
Yeah, it has been written before, but it still worth mentioning. If your players didn't give you a clear description, ask them for more input. While some of the answerers did suggest that you should not ask leading questions, many a time you actually should. From my experience, many of the shortest descriptions for actions come from players who didn't get the full picture in the first place. As such, leading them a little bit (or even more than a little bit) will help them to see the picture more fully for their next descriptions.
Furthermore, you stated that your players are new. New players don't always know what is expected from them and what is not. By leading them a little bit with your questions you actually help them to better grasp what should be described and what not, and to what extent.
Lead from example
You are their GM. They look at you, with their big eyes like you're some kind of a teacher figure and they try to understand from what you do what they should do. This means one simple thing: If you'll describe your actions, they will describe their actions. They will probably start slowly, but sooner or later it will come. Let them see in your descriptions what is expected from them.
Reward for great descriptions
You're using FATE, so use your fate points for your advantage. fate points are supposed to move all the time, to be given and spent very very quickly. This thing enables you to give extra ones without fearing for your adventure/story. One of the players gave a great description? Give her a fate point while explaining for what this is given. "See what Lisa did there? Describe your actions like that and the next Chip is yours…"
The spending mechanics are geared toward describing too. In order to spend a fate point you have to either describe or to be in a situation where one of the aspects comes into play. Build on that, make them add those nifty extra details in order to use those fate points of them. Remind them that by describing they'll be able to unlock those special abilities and possibilities.
Make them swim in details
Although you shouldn't take this literally, do your best to build a picture in their heads of what's happening. As I've mentioned before, many short descriptions from the players come from not understanding and not seeing the big picture of the scene. When they don't know that there's a barrel in the left they won't use it. Make them see the barrel and they'll use it. You're their senses, so what you don't describe doesn't exist.
Let them add small details
The last paragraph is true with one exception: If your players suggest adding a small detail here or there, let them. Sometimes they'll need to spend a fate point, but only when it really is important. If, for example, they're in a combat and one of them asks if there's a barrel at the end of the market, ask yourself if it is logical to find one in there. If so, give it to them. Many a time, with these questions they try to ask you if their idea is good. If you'll give it to them many a time they will come with one of those legendary acts. As a rule of thumb, if it is useful to this particular scene only say yes.
Defending others works the way the first paragraph you quoted says so:
You can even make defend actions on behalf of others, so long as you fulfill two conditions: it has to be reasonable for you to interpose yourself between the attack and its target, and you have to suffer the effects of any failed rolls.
That's it! That's defending others. If you can justify doing so, you can get in the way of the attack, and defend against it yourself, and you take the consequences of doing so.
The extra stuff confusing you isn't the rules for defending others. If you want to defend someone else, this paragraph is all you need to keep in mind.
The section from FC160 is just saying you can take a hit if you want.
FC160 only points out that when you're defending, if you want to, for whatever reason, you can simply not roll at all and leave your defence at +0. That includes defending yourself, or defending others.
It gives an example of doing this when defending someone else from an arrow, but that's just an example. You could also roll to defend against that arrow.
Sometimes, though, you won't be able to. If you really want to save that person from the arrow flying at them, it might be that your only option is to take the hit yourself. Most people can't deflect or stop arrows, and you might not have a shield handy.
The defending stunts you quoted are from the Fate System Toolkit
The Fate System Toolkit isn't there to give you Fate Core/Accelerated material, it's there to help you mess with the Fate system and reconfigure it. The material in that book is there to explore mechanical possibilities, not suggest highly effective character options. Not all options will be appealing under Fate Core's configuration of the Fate engine.
The two example stunts you saw were for that exploration: one was an example of a particular configuration option, and the other was just a stunt on an example character sheet. (Example character sheets don't have to be mechanically awesome!)
Plus, not all stunts are inherently and always useful for everybody. They don't have to be. Many character options will range from useless to stellar depending on your story's setting and tone, the capabilities and power levels of your characters, the kinds of challenges you'll face, and other factors. Different Fate implementations also value stunts differently — while Fate Core allows stunts which grant +2 to something to always apply, Masters of Umdaar limits +2 bonuses to once-per-scene activation because of its different focus.
One of those stunts, though — Dwarven Shield Maiden — looks pretty good to me. A Dwarven Shield Maiden's aspects probably give her a lot more justification for being able to defend against an attack even when others wouldn't be able to. Can you leap in front of an ally and defend them from a hail of bullets at +1? Probably not, but a Dwarven Shield Maiden can.
Defend the Weak could also be okay. Because it operates off of Fate points, it may even leave you able to defend when you otherwise couldn't, e.g. you're bound up and immobile. Doing so may even be a self-compel. This is something that'll vary, as above, depending on the context it's being used in.
Best Answer
Create Advantage: Make opposition more potent
In my own experience (somewhere about 20 games - mostly one-shots, though - GMed) the more potent the opposition, the more players are encouraged to make use of create advantage action. So, for instance, if a bad guy have his defence skill, which he uses to counter any attack action the players use, high enough to require them to use more than 2 fate points on average roll (Like, +6 defence vs +2 average player attack) just to make it a tie, then they'll probably start thinking about making those advantages. Same goes for high enough attack skills, that may deal significant harm to any PC.
I've had once a very interesting conflict, where PCs were facing a very strong enemy - mostly cybogized human with heavy X-COM-stylish plasma gun. He was also backed up by four henchmen. The PCs tried first to just shoot everything they saw only to realise they don't do enough damage by attacking alone (and suffer a few consequences while realising that) - henchmen are using very good cover (creating advantages for themselves) and main enemy is rather resistant to all their attacks - so they started to position themselves and making use of environment to prepare for decent strike. They end up making his henchmen to betray his master and then collapsed most of the ceiling on his head to win, one of the PCs was forced to take Extreme consequence and all of the players were mad happy about this encounter.
Tl;Dr version: For your players to start even considering using the create advantage action you must make sure they can't just shoot ot provoke everything they see without any consequences.
Side note: Be sure to prepare them somewhat before they engage such strong enemy. It's rather not fun to be used as a floor mop, when you haven't got any chance to prepare for it. My players in mentioned encounters were rather aware that the enemy will be quite strong, and in addition they let one of his henchmen escape in the previous encounter, so the baddie was also prepared for PCs.
Overcome: Give them a choice
As far as I remember, the only rule about overcomes in conflict in Fate Core rulebook is the time when you try to move more zones than 2, or something blocking your way and you must overcome it. So other than that, if you don't explicitly provide them with obstacles to move through zone, they don`t even need to overcome anything. So, for you to make them use this action, you have to either make any battlefield they encounter a war-torn bomb-and-shrapnel filled craterfield, or you could give them a choice.
By choice I mean, that they'll have some sort of reason to not just punch their enemy in the face (or make some arrangements for it - creating an advantage). It's usually not something that they can use to win over their opposition - that would be create an advantage action - but it must be something they might consider more important than winning. It might be a lock on the gate, that preventing them from escaping an endless zombie horde, or it might be some valuable shiny they might want to snatch before the pirate ship is utterly destroyed by their war mage, or whatever. It is, of course, might help them in the end to punch opposition in the face, like great artifact sword of awesomeness they spent 3 turns digging from debris, while other part of the party were fighting with incoming waves of goblins. But in general it must provide some sort of alternative goal they might have in addition to just defeat the opposition.
Tl;Dr version: Make something they will want to do other than punch their enemy in the face, and make getting that done difficult - hence, overcome action!
Side note: Of course, making some terrain hard to pass by, requiring overcome action is also a good thing if it provides some sort of tactical choice. My point, is that it is not the only one option to make overcome useful.