I have sewn up many hens whose whole back was open and exposed - skin ripped by roosters. I used regular needle and thread. All did well and feathers grew back and all are alive and healthy today. I did finally buy chicken saddles for the them to wear but that's another story.
My vet did finally give me a box of sterile pre-threaded needle with dissolvable sutures and you may be able to get one from your vet because I now use those and it makes it sooooo much faster and easier.
But, using needle and thread:
First clean with Betadine and wash wound well.
Apply stitches to reattach skin.
Clean again with Betadine. Then apply Neosporin to wound. I also spray with Furall, it's purchased at feed store. It's a yellow antibacterial spray that helps the wound dry and repels flies and prevents others from pecking at wound. But I then also isolate the wounded bird till healed. On the worse of the worse I've recleaned the second day but on most I just let it start to heal and dry. I don't cover the wound. On some that were so ripped up I didn't think they would survive, I gave 1 cc of Pen G after surgery and another for the next couple of days but I've found that none of them ever got any infection and all healed just fine without the shots. I add Terramycin to their water while isolated that first week.
I normally operate on them alone and they just lay calmly till I'm done and then get up and eat a treat. Trick is to cover their head so they can't see and lay them on their side. You can wrap them in a towel if you are only needing to operate on their head, neck or feet and they will lay calmly and can't flap and move around.
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In your case, as posted by others, clean the wound as well as possible and get all scabs and infection off wound before stitching.
