Sewing twill numbers on a football jersey is an easily accomplished task with the proper supplies. This method will work for repairs, if you perhaps have a number that is coming loose, or for adding a whole new number. Numbers are purchased as iron-ons, but require reinforcing to withstand the abuse and constant washings of sports jerseys.
Position your number patch where you want it, being certain that it's centered correctly.
Iron in place according to package instructions.
Pull off your desired thread length. This can be any length, but remember that you will be doubling it when you thread the needle. Too long a piece tangles and knots easily. Shorter pieces are easier to work with, and it will not hurt to have tie off and start with a new piece if you run out.
Thread your needle; double the thread back on itself. Even the tails and knot.
Start sewing from the underside of the patch, so that the knot is hidden between the jersey and the number.
Use an overhand stitch (Reference 1 p. 73) to secure the number to the jersey. Use small stitches, coming up through the jersey fabric and the edge of the number patch and going back down through the jersey fabric slightly beneath the edge of the patch but without catching any part of the patch.
Use this method around all outside and inside edges of the numbers, keeping stitches small and close together.
Overlap your starting point by an inch or two before tying off the thread, once again concealing your knot beneath the patch.
Use a thimble to keep from stabbing yourself when pushing the needle through the thick twill numbers.