There are a lot of youtube videos on making those fabric balls. I made one but not a very elaborate design, just segments like an orange. You will make it easier on yourself to draw it out with a pencil on the ball, then take an x-acto knife or box cutter and cut along your marks. Use a small amount of glue on the segment you're working on, then place the fabric on it, slightly bigger than the segment, and work alternating sides with a thin flat blunt object to shove it into the seam. Trim off excess then smooth the rest into the slot. I have this plastic purple clay tool that I used that had a pointy wedge at one end that worked wonderfully, except when I missed the seam and was damaging the ball instead of shoving in the seam. If it hadn't been for that damage I might not have glued some ribbon over my seams, but the ribbon hid my divet. hehe. So I would also suggest taking your flat blunt object and just running it through your slit before you start trying to shove fabric in. It will make it easier.
I loved the santa undies when I posted them, but I love them even more with those adorable cute clothes pins. I have some... more reason to want to make them
