Guavas blended with water and strained, or with milk if you prefer it that way. Ten minutes, five glasses, and the seeds are what the straining is for.
Straining is not optional with guava. The fruit is full of small, very hard seeds that a blender does not break down, and they are unpleasant against the teeth. Thirty seconds of blending and then a sieve, pressing the pulp through with the back of a spoon so nothing but seeds is left behind. Water, milk or both. That is the choice this recipe offers and the two give quite different drinks: water keeps it sharp and light, milk turns it into something closer to a smoothie, sweeter and rounder. A mixture is what many Colombian households do. Ripe guavas smell strongly through their skin and give slightly under a thumb. Unripe ones are hard, astringent and need far more sugar than the drink can carry.