Yes to all of the above advice. It helps to work on the edge of a table and sit on a low stool so that you can tuck your head under and push the needle up from the back where you have made a hole, all without disturbing the jersey as it lays on the matboard. If you find it hard to push the needle through the fabric layers and matboard, sometimes I'll gently tap it through with a hammer. Yes I ruin needles this way, but it gets the job done. I also prefer a leather finger cuff over a thimble whenever I'm sew mounting or even pinning extensively because you can retain a little bit of grip. It also helps to aim primarily for seams, as having been sewn already, it's easy to hide stitches in and you won't make as prominent puckers where your stitches fall. Lately, I've been taking a very easy route and have been making a foamcore insert that fills out the shirt as much as I want it to. Then I wrap any excess around the back and pin it in place with some of my nickel plated brass pins. That way, it has a nice smooth and taught appearance. I then stitch the whole thing down, again primarily around the collar, on the sides where the sleeves are going to cover my stitches, and along any seams that run down vertically. If the sleeves need stretching too, you can make individual foamcore inserts for them as well, stitching through those as well.
This Blier jersey has a simple square piece of foamcore in the center. I folded the sleeves over after and simply stitched them where appropriate.
This jersey, on the other hand, had a bit more shape to the main piece of foamcore, and then triangle-ish pieces inserted into the sleeves as well.
These two also had inserts in both the body and the sleeves.
These ones, I regret not having a better picture of. They were sewn down without any form, so they are quite a bit looser. I don't know if a form would make the red one look better anyways, especially due to the elastic in the sleeves.
