I had done a number of birds and animals as a kid, all badly!
A couple of years ago a did a squirrel, which was ok, but the paws were really tricky. I did a hare, which was reasonably easy, it was large enough. I then did 3 roe shoulder mounts, 2 were really good, one the form was way off in size, and i was never really happy with it. it was also in velvet, and though it was a success, it was only ever ok. I then did a red stag, in some ways easy and it is thick skinned and large, but also heavy and difficult to work. The guys who do this professionally have a hard job with these fellows, and i vowed never to do one again. (unless i get a really good one!)
One of the experts will advise here soon, but as an amature i find using the best freshest specimen is vital, i dont have the skills to do repairs. skinning really carefully and taking your time with this is vital, get the skin clean and thin. Taking lots of measurements and of close up photos. Get the correct size of form, and get it all trimmed and sanded before you start skinning. Ears lips and eyes are difficult, take your time here. Buy good eyes and clay. I use a liquid tan, it is a beginners way but works for me. I use lots of skin glue, and offer the skin up a good few times before setting it and stitching it. I use heaps of pins,and check it daily for a couple of weeks as it dries. The temptation is to take the pins out and look at your deer, dont do this, leave it.
Once it has been dry for a month or so, so about 2 months since mounting paint in the eye, ear and nose skin with natural tones, look at the pictures you took.
That is how i do it, the pros will do it totally differently, and way better. Most of mine are with friends, 2 are at home and one is in the Stag pub in Aberdeen.