There are several good books on tanning hides. You might want to check them out. First decide if you want to brain tan the hide, or use some commercial product. Tandy Leather sells " Tannit" Which works well, but is not everyone's cup of tea. If you didn't save the brains from the fox, go to your local butcher and ask him to get you some brains from a calf, or hog. Brains are brains, and they all can be used for brain tanning. Or you can use egg whites instead of the brains to tan the hide. After fleshing the hide to remove the thin layer of shinny skin from the underside, you make a solution of the brains and distilled water, and then rub the solution into the flesh side of the hide. This takes time, and you want to get the solution to penetrate as deeply as possible into the leather.
Then let the hide dry. Next comes smoking, which is how you get the hide to keep from rotting or suffering from milddew in wet climates. Finally, you soften the hide. You can do this manually, or you can put that small hide in one of the net bags sold to put nylons and delicate fabric clothing in and put it in your dryer. Put a couple of old( clean) tennis shoes, and or some tennis balls( also washed and cleaned) in the dryer and let the beaters of the dryer, and the shoes soften the hide using a cold dryer. Brush the fur when you take it out of the dryer. Use some fabric softener sheets to soften and get any knots out of the fur, and brushing the fur will produce a very nice fur hide to display, or place on the back of your couch.