A couple tricks I used that have greatly helped improve the quality of my deer hair flies (they used to be awful).
Make sure you use a heavy thread like monocord or 3/0 thread. Rather than securing the thread at the front of the hook and creating a base that will cover the shank when you want to spin, I secure the thread in the middle of the hook so I can leave the hook shank bare where I want to spin the hair. Covering the shank with thread will create friction and resistance when you try and spin the hair or it will cause the hair to spin all twisted.
Cut a bundle of deer hair no larger than the diameter of a pencil. Anything larger than that and you can't secure properly to the shank and anything less than that will secure to the hook without spinning. Holding by the tips and using a fine comb, comb out all the fuzzy crap. This will also help straighten and align the hairs.
Pinch the hairs together and place on top of the hook shank and make a loose wrap over the bundle where you want the initial tie in point. Make a second loose wrap over the same spot and when you are pulling the thread under and then up going to the third wrap, start to apply pressure to the thread and you will see the hair start to flare. As you wrap the thread over the bundle for the third time slowly let go of the bundle and continue to wrap the thead tightly. This will spin the deer hair nicely around the shank with straight fibers. Take about 2-3 tight turns and the bundle should be secured.
Depending on the size of the fly, you may need to add a second bundle in the same spot. Simply repeat the same process.
If you want to add length to the flared head, pull the initial flared deer hair back towards the rear of the hook and make a few tight wraps with the thread immediately in front of the head. Cut more deer hair and comb out except this time cut the tips so you basically have a short bundle of deer hair butts. Using the same tie in method, tie this short bundle slightly in front of the initial bundle and keep repeating until you get the length head you want. The reason for clipping the tips is you know that anything with tips is suppose to stay and anything with butts is supposed to be clipped and trimmed to shape. If you used the full hair for additional length it would be a mess and inevitably you would cut the wrong tips and the fly would look like crap.
Lastly, when you tie off, pull the flared hair back and make 4-5 tight wraps of thread right up against the front of the flared head. This will lock and help secure the deer hair head from slipping foward or backwards. Tie off and apply a little head cement.
It takes a little practice. My suggestion would be to forget about trying to tie a complete fly, practice this technique on a few bare hooks until you get the feel. Deer hair is cheap and it's one of those things that when you get it right (which shouldn't taketoo long), you will know it.