Did a quick search here and found that I had already posted a recipe for making quark: ISO additions to cream cheese or quark
and, of course, make sure everything is really clean, since you only want to grow the good micro-organisms.
If I let the quark drip long enough that it gets too dry, I add back some of the whey.
I once tried shaking the sour cream to mix it. The milk didn't clabber, same as the time I used a whisk. It doesn't seem to like violent agitation.
The short cut method is just to use straight cultured buttermilk, but buttermilk is stupidly expensive in Canada, so I don't do that.
BTW, if you do the culturing part, and skip the oven part, and use cream that is at least 30% butterfat, you get creme fraiche.