Eat it quick before the infection spoils the meat.
seriously:
Strap cat down or have freind hold it, wear leather work gloves, youdontwant scratched by a stray. If you have weed, get the cat high. If youhave hard liquer, give the cat a thimble full.
now
Starilize all open woulds with betodine. Look at the opposing leg and feel it up, you need to know how its supposed to look and feel. Now thast you have some idea, set the bone. DO NOT just push it in. Grab both ends and pull untill you can push the bone in w/o it rubbing the broken edge. You will need to brace it IMMEDIATELY, so have your bandange and splint ready. you will need no less then 2 rigid splints. Check bandages daily, replace them when they become any color other then white.
If the wound looks infected and you dont have antibiotics, ge the cat high again, REAL high. shave all tthe hair off the leg fromt he wound up. get a butter knive glowing hot, and sharpen and sterilze your favorite hatchet. Amputate an inch or two above the wound. Use the hot knife to carterize the wound. Betodine it all and bandage it up.
or
Take it to the humane society so they can kill it.