I think, as with any deity, it is important to first read up all you can on Fenrir and get to know who he is and what he is about. In Fenrir's case, I also would like to point out that he is not a deity one can really "work with" (at least not closely) simply because he tends to be a deity that is not interested in people nor their honor/lack of honoring of him. However, that's not to say that you can't learn from him. Here's a website with some information you may find of use:
I "work with" Fenrir on some level, and often include him in some of my workings (like berserkergangr). I have built him an altar, which I crafted out of a fallen tree stump. He doesn't seem to have much of an appreciate nor car for material things, so if you choose to make an altar or the like- it's more for you than for him. I do devotionals to him by giving him offerings, which can vary from raw meets, blood, animals I've hunted, to physical labor, work, or the act of horsing.
If you'd like to chat about it sometime, you can shoot me a message.