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.
This is what it will look like now, as you can see mine are being shortened by quite a lot. The line of pins will be your finished length.
Next you want to cut away the excess, if it is a lot you can cut it straight off like I do here...
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...
If you need to add more pins once you have folded it to keep your fold in place but don't confuse them with your original line, this line ensures your hem is nice and straight as you hand stitch.
Yes, you are going to hand stitch the hem, some canvas trousers and of course jeans have a machine hem but dress trousers usually need an invisible hem. There are fancy machines like the ones used in factories that give an invisible machine hem, but they can't be that good or why would hems on store bought trousers always be coming down?
Right make a good knot and hide it under the turned up fabric, do a few stitches there on top so it is secure. Now put your needle through only catching some of the fabric on the wrong side ( don't go right through) angle your needle to the left and come up through the hem as in the picture above. I am right handed so I work right to left, if you are left handed just reverse it. Pull the needle all the way through now and repeat all the way around the hem taking care not to pucker and keep the stitches even. Finish with a good knot, what I tend to do is push the needle through like in the picture above then wrap the cotton around the end of the needle a couple of times then pull it right through and that gives a good strong knot.
When finished all your stitching will be invisible, they now just need pressing to make the hem crisp. Either do this on the wrong side or put a piece of cotton over the top to stop them going shiny.
I hope this has inspired a few people to have a go and was not too confusing, I often get asked by friends and family to shorten trousers so I know this is a common problem for many people. Don't be discouraged if your hand sewing is slow at first, just take your time, you will get quicker.