The peel is certainly edible, it's up to you whether you use it or not. If you choose to not peel them, try using large, juicy guavas.
The seeds are edible as well, but perhaps a bit annoying in juice. So perhaps you could put it through a sieve after it's juiced (when using a juicer). If you don't find them annoying, there is no problem in leaving them in.
Some varieties of guava have a strong odour. If you dislike this, you can boil them first, to reduce this.
As for the blender, I've never tried it myself, I've found this website that gives a good how-to.
It says to chop up the guavas, put them in the blender with some extra water (the amount needed will depend on how juicy your guavas are, so don't put too much with them in the beginning), blend, put everything in a clean towel or a cheese cloth and try to press out the juice, so the pulp and the seeds stay behind. There is nothing wrong with the pulp, you can let it in if you don't want to do the towel-step. But your juice will be more like a smoothie.
A juicer would be less work, but it's more expensive. Perhaps try making juice with a blender, and if you really really like it, you can always buy a juicer if you know you like it.
And just a side note: you surely can can or freeze the guavas, so that you don't have to throw them away.
Best Answer
You can buy fennel seed extract, but I can't find instructions to make it. I assume you could just crush the seeds, put them in vodka and strain in a week or two.
If you don't want to use alcohol, you could also use propylene glycol or glycerine.
Another thing you might consider is using vinegar the same way. That could certainly be an interesting accent to certain drinks.
Any kind of attempt to juice the seeds isn't going to work. Extraction should work, I'll experiment with it. I'll update this post in a couple of weeks if no one beats me to it.
EDIT: It says on WIKI that you can make a tea with fennel seeds. By golly it works! I used slightly crushed fennel seeds and added boiling water. Once it cooled to drinking temperature, I strained it. I'm drinking it now, it tastes like fennel seeds. It's quite nice with a touch of honey.
Later EDIT: I just checked on the fennel seed extract in vodka and the vinegar. Both seem to be working just fine. I think the vinegar has some real potential. Maybe not so much with fruit, but perhaps with vegetable juices and/or kefir. I'm also thinking that it is going to make a great salad dressing.
UPDATE: The extract made with vodka is exactly how I expected it to be. It's a bit boozy (of course) but it tastes like fennel seeds. A few drops of this could easily replace actual fennel seeds in any recipe I can think of.
The vinegar is another story entirely. This stuff is awesome. I'm not yet sure exactly how I am going to use it, but I will, and I'll make it again. Furthermore, this has inspired me to make an infused vinegar using all of the whole spices that I still have from my most recent adventure with pho (Vietamese noodle soup). I can imagine it as a secret ingredient in all kinds of goodness. Thanks for the question which in turn inspired good stuff!