I didn't take any pictures while I was making the real thing but I sat down this weekend and ran up a little mock-up band ... basically it is the exact same procedure as if you are adding a neckline or armhole band to a Tshirt but just on a much smaller scale!
Firstly, when you cut your Tshirt or bodysuit pieces cut the sleeves long enough that they will cover over your hands, as above. Make the Tshirt.
Try on and mark with a pin where you want the thumbholes to sit on the sleeves.
Now, measure around the base of your thumb and cut 4cm wide bands to fit, adding 0.5cm seam allowance onto each short end.
Pin from the band side, since stitching is to be done with the band side up.
But because there are almost double the number layers of fabric sitting at that joining seam point compared to elsewhere along the band this area is can sometimes be a bit bulky and you sometimes end up with a slight bump no matter how carefully you guide that seam through the machine. Seriously, getting a nice smooth and even-width band all the way around is often the hardest part about making a Tshirt!
So lately I've been cutting and sewing the join on the diagonal, reducing the number of layers at the joining seam, and I think it's been an improvement with almost no bumpy bit at all!!
It is a particularly good idea for bands like a cardigan button band, on which slight bumps and irregularities in thickness and width are particularly noticeable.
Later edit in response to an email question; just to be clear, the band is NOT a bias-cut band, it is still cut cross-grain just like you normally would and it is just the short joining ends that are being cut on the diagonal :)
