I mainly find a block or a piecing idea and then go with that method. I don't make quilts that have the same fabric in the same blocks, all sewn together to make a top. Every block is different fabrics, even if the block is the same design. The size ends up being what I want, so buying the amount of fabric is always a guess, of which I usually get more than what I'll need. I have made 4 OBWs, and that is buying 6 repeats of the print plus some extra for a border and the back. That is a technique of cutting repeats and piecing together hexagons cut from strips. It totally changes the print to a whole new look.
I've also made 4 PP quilts and those each are based on a pattern, but I used my own choices of fabrics, so the quilt becomes my own.