How do you make guava juice?
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.