The hardest part
The part that I think is the hardest for someone just starting is getting the paint the right consistency which takes some practice.
Once you have the paint just right simply rotate the rim on the hub or have some else rotate the rim slowly and you should be able to get a nice line.
If not simply wipe off the One Shot paint and start over again.
Again a very important part is getting the One Shot paint to flow consistently and the there are a number of different ways of doing this.
The difference between someone just learning pinstripping and a veteran is how long a line they can pull without needing to more paint on the brush.