Having sewn hundreds of dogs I would NEVER use a big needle. Smallest needle you can use makes the smallest holes/tears.
Run them through a washer with typical fabric soaps. Tumble dry without heat.
Sew on the stretcher with short pieces of clear "Invisible Thread" which is very fine mono, probably under 3#, and available from most fabric stores. Sew from well back from the edge of the hole and run the needle right through the hide as much as possible.
Flaps often get turned around so check very often as you go on both sides, part of the reason behind using short pieces of thread. Keep the seams touching edge-to-edge, never overlapped.
Invisible thread makes for tiny knots but it can also cut the hide, so be careful when pulling tight. That control is another reason for short pieces of thread.
Save pieces of the non-fatty, clear membranes cut off during fleshing to lay over the repairs. Not too much, just enough to get the patch smooth. Done right they are nearly invisible.
Be careful when turning the hides on the stretcher that you do not tear your repair. Once dry they are quite tough.
Make sure the hair is very dry before stretching the first (hair-in) part or you might find hair getting loose at the ears. I check the area often and if I find loose hair I do not panic, just use a hairdrier, again, gently to dry the area. Once dry the undisturbed hair will not slip.
