I can't help on how wide to make your fur pieces or where to put them, but I can give advice on working with fur, based on working with shearling, rabbit, and some goat (think a smooth-haired dog coat) on leather projects:
Fur has grain. Plan your pieces accordingly. You can use the grain of the fur to your advantage to hide piecing.
Cut from the back, carefully, with a sharp knife. You want to cut only the skin, not the fur itself. Even if your fur is short and doesn't seem to have much grain (like shearling), you still want to cut it this way. Some people do use shears, but carefully, so they don't cut the fur, but I prefer using a knife. If you have really thick fur, it may try to hold your pieces together after cutting through the skin.
Circling back to the grain issue, depending on your fur, you may find that the fur will completely hide the cut edge where the hairs hang over, and looks fairly thin/bald on the other edge.
Leather doesn't ravel, so hemming isn't strictly necessary. But if you do want to turn under a hem or in a seam allowance the fur is causing you bulk issues, you can carefully shave it out of the seam allowance with an electric beard trimmer or sharp scissors.
For machine-sewing, test with strap material first, use a leather or denim needle, and use paper under or on top of the hair.
For hand-sewing, do not wax your thread, or polish your thread after waxing with paper. You really
don't want it sticky. If possible, sew so that you are coming out of the skin from the back, kind of with the hair. You'll still end up with some hairs caught in your stitches, but not as badly as if you were coming in with your needle through the fur then the skin.
Fur will hide a lot of imperfect sewing, once combed/brushed/eased out of the stitches.
If you need to join two pieces, and the fur is thick enough, you can just butt the edges up and use a baseball stitch.
Check your library for "The Art of Hand-Sewing Leather" by Al Stohlman. A lot of leatherworkers will also have copies. He has a section on sewing shearling and fur.
If the skin seems weak, and you need to join pieces or want to be able to remove the fur for re-trimming later, consider sewing the fur to a fabric backing, then through the fabric only onto the garment.
Take a look at how the coat that's going to become fur trim was constructed. It will probably give you an idea of what works for that particular fur.
For fur trim, I wouldn't turn under the edges if I could avoid it - I'd sew it down in place with whip stitches over the edges. You want the grain to go from your head to your feet, like on an animal. If the fur coverage on the "head" edges of your pieces around the hem and cuffs is thin, an alternative to turning the edge under, which may not be a good effect depending on the hairs, is to cover that with some additional fabric-ish trim.