Thanks everyone!
I used some circle templates that Patrice sold a while ago for the size of the pincushion. The buttons are hand sewn after completion. I love buttons and collect them all the time, just thought it'd be a cute addition but it's super cute without them if you don't have them on hand or prefer not to hand sew. I would say that the bottom isn't completely flat, it mimicks the top with a solid piece of fabric and center small button. I made some similar to these quite a while ago and will see if the pix are still available in the photo section. I have no idea what I called them but the center buttons have thin ribbons coming out to divide the circles into sections. If I cannot locate, I can take a pic of those, too but they are not made from selvage squares.
All my best, RETRO