There are a few ways to shorten jeans. Cutting the stonewashed section off and reattaching is one option. However in this video I explain that you can also encase the joined section so that it is neat and tidy on the inside.
I have not included a step by step on this video, because the full video is exclusive to my Magic Circle online tutorial at geniecentre.com. However it gives you the idea and if you are interested, the video explains where you can find this information.