Go to
www.trapperman.com there you will find the answers to all of your questions and then some. What size and type of leghold do you have? The most productive way to trap foxes is with the regular old dirthole set. If you are serious abouttrapping them buy some good lure. I used one made by Carman's but can't remember the name. There are lots of modifications you should make to the traps before using them.
I caught a couple of foxes using scent post sets. Get some fox urine from your local sporting goods store. Find an area with fox sign; tracks, scat, trails, fresh kills, etc. Next, get down on their level and try to identify something that sticks out. It could be a post, clump of grass, a small rock, etc. Spray it with the fox urine; a squirt or two will do. Set your trap 6"-8" from the scent post in the most likely direction of travel, and make sure it is bedded and covered properly.
Be sure you modify your traps, bed them properly, set your traps on good sign, and be scent free. I really loved trapping, but don't have anywhere to do it anymore. Good Luck and keep us posted!