I find pictures I like online and then make up my own pattern. This one I wanted rectangular instead of square, so started out with rectangles and went from there. The block is about 12" x 16". I may use it as the front of a machine cover, so didn't have any exact sizes to worry about.
I have a lot of experience sewing, but not much with quilting, so I'm in the "learn by making mistakes" phase. I'm not against buying patterns, but so far what I've done has been easy enough to figure out.