Fold your jeans over in half with the backside facing up. Then fold the leg on top 90 degrees out at the knee. From here you can start rolling the jean from the waist down to the end of the legs. Next wrap the leg hanging out over the rolled-up jeans.
How to Fold Fold Jeans (Quick Ways to Save Space)
I recommend watching the video for visual aid, as the process is much easier to understand.
I have also added a few more methods to fold your jeans so that you have some options depending on your jean type.

Check out this article for more ways to fold pants, and this article to learn how to fold your clothes into rolls.