You can't. This is a serious injury that REQUIRES a vet. If you can not afford one, then you need to do the humane thing and put it down. Unless you are a vet yourself, you do not know what you are doing. There are many things that can go wrong with an injury like this. Take the cat to the vet.
If u want the utmost proper care for ur cats leg and it's broken the best and I think only solution is to take him or her to the vet since they know what they are doing and will help ur cat hope ur cat feels better soon
You can't take your cat to the Vet, reasons best known to you, I would suggest you must any way, you can wrap the broken leg with cardboard and put adhesive tape around it for at least 15 to 20 days. Try to give calcium tab in milk, it will heal its bony injury fast.