Not a very difficult task. Anyone with a bit of sewing skills and a hot iron could do this. I thought it might be helpfull to show how to do this with the trousers in my collection. I shortened a pair today and made some pictures of the process.
Step 1. Put on the trousers and ask somebody to help. Easiest is if you can stand on a chair or little step. Fold the trousers upward and pin to what length you want them to be.
Another option is compare a well fitting pair of trousers (not jeans) and measure the inseam.
The next steps I will explain with pictures.