First, ride with Epona in Milk Road. Make sure you set the Fierce Deity's Mask on any C-Buttom. Nex, stay on Epona and go the bird statue thing and save your game there. Then, open your saved game. DO NOT PUSH ANY BUTTOM! Go to the Gorman Bros and race them. When you're on track at the countdown, press the C-Buttom where the Fierce Deity's Mask is. You be Fierce Deity Link now, but don't do anything. If you press start the game will freeze. Let the Gorman Bros win the race, then after the race you should be Fierce Deity Link now. However there are some bad things you must know playing as Fierce Deity Link outside of boss battles:
1. You can't change back.
2. When you're in Clock Town, you can't get out even if you talk to a guard you still can't get out.
3. Don't talk to the Bomber Kids and the guy at the octorock shooting place in Clock Town.
4. Don't go into Stock Pot Inn. You will be trapped forever.