I’ve made quark several ways but this seems the best (and one of the simplest).

1 1/4 gallons buttermilk
1/4 tablet Junket rennet or 2 drops liquid rennet
1/4 cup cool nonchlorinated water

You can easily make the buttermilk by adding one quart of buttermilk to one gallon of homogenized pasteurized milk.  Let it sit at room temperature (66-72 F.) for 12+ hours until you get a thick buttermilk.  Do not let it sit longer.  When you get to that point dissolve the rennet in the water and add to the buttermilk.  Let it sit until it firms up.  Then hang in cheesecloth for 12-24 hours until you get the desired texture (close to sour cream).  Delicious!