Map Awareness
Riot has garnered some pretty effective videos with their Video Tutorial contest (Links posted below). As a beginner, you want to primarily focus on making sure you don't get ambushed by the other team (called a "gank"). While some of this involves Map Awareness, you also need to learn to not overextend, as well as keeping an eye on the enemies you're fighting in your lane. If you cannot see one or both of the enemies in your lane, let your teammates know!
Last Hitting
Killing minions and enemy players nets you gold to buy items. However, if you don't land the killing blow on an enemy minion, you will get ZERO gold, even if you've done most of the damage to it.
The faster you kill enemy minions, the faster the minions reach the enemy tower, which will make quick work of all non-cannon minions. It's better to keep the minion fights as close to the middle as you can; you only need to finish off the minion to get gold from it -- see if you can limit yourself to only attacking minions when you will kill them!
Tower Aggro
The enemy towers are dangerous. They have lots of health, armor, and a powerful attack that can quickly decimate low-level heroes. It's important, then, to know how the turret picks its target.
Enemy towers will always target allied minions first, switching to champions only if there are no longer any allied minions around. The exception to this is if you deal damage to an enemy champion, in which case the tower will immediately switch to the damaging champion, shooting at them until they die or move out of range.
Let me repeat that: If you deal damage to an enemy champion within range of the tower, it will immediately start shooting at you: You do not want this. Be very careful when attacking the enemy at their own tower.
Recommended Viewing
Four of these videos are a result of the Valoran Video Contest held by Riot Games, and are quick, 1-minute videos touching on just the basics (which seems exactly what you're looking for). The fifth video is made by Shurelia, an employee of Riot Games, and goes much more in-depth into what "Zoning" is, and how to take advantage of it.
Best Answer
This is a known bug with Ultimate Spellbook. Ekko's ultimate has a serious bug, which is really unfortunate because it's such a good ultimate.
This bug was reported a couple of times on the Patch 11.24 Bug Megathread on Reddit, a thread that many Riot employees watch closely, so we can hope the bug will be fixed soon.
In the meantime, I (and others) have determined that the bug occurs for non-Yuumi champions when the Ekko ultimate comes off cooldown while dead. When you revive, you won't create the Ekko ghost behind you, and when you use Ekko's ult, you'll enter stasis for a short period of time, but you will deal no AoE damage or teleport anywhere. For Yuumi, this is even more broken, as the ghost disappears whenever you attach to an allied champion (as you noted).
There's no real way to work around this when playing Yuumi as her entire kit revolves around being attached to champions with her W. You can work around this bug when playing as any other champion, though, by immediately using Ekko's ult as you respawn if the ghost wasn't created for you. If Ekko's ult comes off cooldown while you're alive, it should function correctly.
Other than that workaround, there's nothing we can do to help you but advise you to not pick Ekko's ultimate when playing Yuumi and wait for the bug to be fixed.