Move the left stick
(or vice versa) in quick succession and your goat will start 'manualing' by walking on its two front or back legs. It's also best to try to perform the manual on a straight stretch or wide open area. To safely land the manual press
to jump out of it once you've hit the required 3000 feet. I'd also recommend putting on slow-mo mode which is defaulted
This makes things infinitely easier.