I have this disc and have made a pillow for a friend with it. I trimmed the blocks 1/4" from from the stitching line that outlines the block. Then when I stitch them together I stitch barely inside that stitching line so I make sure the squaring stitches don't show. The blocks fit together very nicely. As for the interfacing, I use 911 fusible from Joann's. It's light weight and often on sale. I buy it by the bolt.
Hope this helps.
Kerry