If you have other trousers that are the perfect length you can skip trying them on and work from that pair. If this is the case simply lay out the good fitting trousers, put the new ones on top (making sure everything is lined up) and mark the new finish line with a line of pins on the new trousers. Also be sure they fit correctly at the waist because if they are drooping due to being loose this will affect the hemline.
Now if you don't have a pair that are the exact right length or you have new shoes that you will be wearing these trousers with here is what you do.
Try on the trousers with the shoes and get a friend or family member to help you turn up one leg and pin. This may take a bit of trial and error but don't try to do it yourself because as you tip forward you are changing the hem length. You only really need to get one leg done then you can take them off.
Next lay them out on a flat surface and take up the other leg to match, once you are happy they are the same you are going to place a line of pins along the bottom of the fold, this is your finished length.
You can now take the other pins out ( the ones that are holding up the hem) and let the hem fall back down.
You will want to finish this raw edge so either over lock or zig zag stitch, I used a zig zag on number 7.
Now you need to fold back your hem so that the pins are at the bottom of the fold...