In this video I teach you how to do the Triangle submission from the Mount position. I really like this setup because it is easy to do and a lot of people arent expecting it.
After practicing this move you will find that the faster you swing the leg around the neck the easier it will be. There is a little time between learning this move and being proficient at it where it will require you to step with your foot on the ground between the mount position and the position with the leg around the neck (watch my first rep, I stutter step before catching it in the Triangle position). After a few reps it should really be easy to get that leg around the head.
Make sure to adjust your base and lock that leg BEFORE you roll. When you post that hand far out to the side you will notice that it is far beyond what your opponent can reach, you shouldnt have to worry about them grabbing that arm to challenge your base.
Keep heavy on your butt, squeeze your knees and pass that arm in front of the chest to make the Triangle tight. If you have problems locking your legs or passing that arm there are plenty of options but none of those options involve you rolling to your back to lock on the Triangle. Please dont do not give up the position in hopes of finishing.
I show one simple way to maintain that top position but there are many acceptable transitions. Remember the saying "position before submission" and you cant go wrong.