I hand sewed mine onto muslin squares but for the centes to get them nice and round I had my son cut me a piece of metal flashing into the appropriate size cirle, I stitched around the edge of my piece by hand, put my metal cirle on and pulled the thread to make it a snug fit, then I pressed it and removed the metal piece, I tried other methods but this gave me the crisp edges I wanted. Depending on your center size you could use a canning lid or some other flat metal piece. hope this helps, I am ready to put my sashings on, Mine are only about 8 inches across so I had to make 72 for a queen size quilt. I assume yours are larger.