This worked for me :
- Level 3 Trust at Diamond Lake camp to get Nitro 3 on bike.
- I downgraded bike to Engine 2 and Standard tyres (less grip)
- Fast travel to Hot Springs and go just outside to the Hotel, nice section of flat road.
- Stick full left or right and accelerate with R2 then tap circle you will now perform a perfect Doughnut (Trophy WILL NOT pop yet)
- Adjust left stick slightly until you increase the size of the Doughnut
- Press and hold X for Nitro and wait for trophy to pop
Hope that helps someone