Margaretta Harris's block has a piecrust edge. I know this because Barbara Brackman used a photo of the original block in her post on Pumpkin Patch blocks.
Although this block wasn't especially difficult I had to figure out how to get the pattern onto the fabric. The concentric rings all completely separate from each other and I wanted to use a single piece for the whole block. I don't have any how-to-applique books so I usually try to figure things out myself (with a little help from my friends).
I used a large business size envelope to trace the pattern onto. The envelope already has one fold and I folded it once again. I cut around the outside of the pattern, and also the inside circle. I drew around those two edges for turning under line.
For the next circles I cut between the two turn under lines and traced the cutting line on my fabric. That meant less lines to draw on the fabric. I wanted to needleturn the smallest channel I could so I didn't mark the line. I cut one section at a time, traced my cutting line, then did the next circle.
Before I basted the red to the background I cut slots along each circle. I cut a slot, left a space, cut the next slot. I left plenty of fabric so the pattern did not distort. The cut slots gave me a space to slide my scissors under and complete the circles as I sewed.
I hope these pictures assist you and your block. You may approach Block 20 in a completely different way, if you do you could leave a comment about your methods. There are no quilt police on this blog!