Others have given some very good advice, but here are the concise "rules" on treating large wounds.
1. Flush with warm soap and water or saline solution each day to remove accumulated bacteria and dead tissue. This will prevent infection.
2. Keep wound moist and do not allow it to dry out and scab over. This will encourage the growth of new tissue.
For number one, you may use other things to clean the wound, but saline and soap and water are the least expensive and probably work the best.
For number two, you have a wide leeway in selecting the materials to achieve this. Plain Vaseline, antibiotic ointments, Manuka honey, and Silvadine all will inhibit bacteria while keeping the wound moist.
Trying to bandage a chickens body is tricky and chickens hate the idea. They will usually keep at it until they manage to remove the bandage. Vetrap, the name of elastic bandage that sticks to itself, is probably the best to use on a chicken, but I advise placing a non-stick gauze pad over the wound and using the Vetrap to secure it in place.
You have the option of leaving the wound unbandaged if you can keep the patient in a clean environment during healing. Chickens heal up rather miraculously from even the worst wounds, but patience is necessary. It can take a while. I wrote about treating a small chick with a scalped head here.
https://www.backyardchickens.com/articles/healing-a-severely-injured-baby-chick.71976/