Sharon Schamber has some Youtube videos that show how she does this for blocks that are too small, too large, or wonky. Here are links (not necessarily in order):
https://www.youtube.com/watch?v=v-3RIWhBvcA
https://www.youtube.com/watch?v=i6aplw_tVZc
https://www.youtube.com/watch?v=lIjZqABo2NY