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.

I just made my morning glass of juice using the golf ball sized "baby guavas" from Mexico. I used a centrifugal juicer from Breville on speed level 3 and just washed the whole guava. I threw 1 guava into the juicer whole along with 1 peeled tangelo, 1 peeled naval orange and 1 whole Granny Smith apple. The juice is delicious and a beautiful colour of yellow with no identifiable seeds or pulp. The guava taste did not stand out from the other fruit, so I will increase the amount of guava in the future. The juicing machine may be costly but it is the most efficient way to produce juice. The whole process took about 5 minutes, including the washing up of the machine

Since its quite a squishy fruit I doubt it would turn into any thing more than a guava purée in the blender. Likewise for a juicer even (unless you choose a firmer one) where it usually uses a spinning blade which grates the fruit and a spinning compartment to spin the juice out, this would just create a purée/coulis like it does with soft berries.

I would blend the fruit then put the pulp into a muslin bag, hang it over a jug in the fridge and let the juice drip out overnight.

Alternatively you could turn the guava pulp into a sorbet, I bet that would be delicious!

You can also use a food mill. I have never peeled guavas, but I cut them in half, scoop out the seeds with a teaspoon, chop them up and either stew them for a few minutes, some sugar if required, and then they can also be frozen at this stage, or put in the 'fridge' to be eaten at breakfast or as a dessert with vanilla ice cream.