The best tips I know are put the larger, fuller piece of fabric on the bottom so the feed dogs can help ease in the fullness. It also helps to pin at both ends and the middle and then keep dividing up the fullness that still exists the same way. If you are doing something like the flower block that is actually gathered you could baste the larger fabric and actually gather it before you attach it to the straighter piece. The most important thing to remember is that fabric is not a finate medium. If after all your efforts it still doesn't lie flat steam the heck out of it.