I didn't use flats with DD cos they were too bulky on her. I can give you a tip on what I did with prefolds though and they worked great.

Fold it in thirds. Fold out the back half to form "wings". Fold the front bit up towards the middle to form a little pad.

Then, I wrapped the wings around her waist and secured them to the pad bit with a snappi.

And get a good cover. A good fitting cover will save everything - preferably with gussets like a mother ease cover. Can't go wrong with one of those.