Hi i am a forza Horzion drifter and i can drift pretty good on horizon but when i try on forza 5 i just cant do it if anybody could help me i need good cars and goods tunes and some drifting buddies 
Try starting with something like a 240sx with lower horsepower. I would say around 250-350 horsepower. If you try to go more than that you will only spin out everytime. Tuning is also a BIG player in how well you drift. The right tune could be all you need
I can help you out with the buddie part lol but as far as advice im like you I can drift on all the other forzas but fm5 is a hard one I would ask drxsickness he helped me out ALOT hope you find what your looking for man good luck =)
Snow is right,
Start small. A low HP car on a small coarse will be best to start out.
if you are needing help with point drifting click link below i have step by step tuning setup Videos for AWD and RWD might take a while to find them on my videos but if you are looking for help on tandem drifting then it probably wont help