Heat the milk slowly in a large pot to 185-190 degrees F.
Turn off the heat and stir in your acid – vinegar, lemon juice or lime juice – a spoonful at a time, stirring, until curds form and separate from the yellowish whey. Let it sit for 10-20 minutes.
Set a colander lined with cheesecloth over a bowl and strain the curds from the whey. Discard the whey or keep it for other uses.
Add salt at this stage, if desired, and stir.
Stir the curds up a bit and let it drain for 10-20 minutes.
For firmer cheese, wrap up the ends of the cheesecloth and continue to drain an hour or so. For dense cheese, place a weight over the top of the wrapped cheese (or cheese that has been placed in a cheese mold) that will press it for several hours, until it is packed.
Queso blanco will keep for about a week in the refrigerator, though it is best when used immediately.