Also I should note that there are many ways to hem pants, and this is just how I happen to do it. I am lucky have a machine with a special blind hem setting.
First, put your pants on and roughly pin up the hem to where you want it.
Take them off, measure how far up your pin is, and then mark that length all around the pant leg. In my case it was about 6". Once you've made a ring of chalk on your pant leg at the 6" mark, pin it up and try it on again just to be sure.
Sidetrack! While ironing my pants on my ironing board, I set down the iron on the rug and it lost its balance and fell over onto a plastic shopping bag. Oops.
anyways, this stuff was nice because it came with a terry cloth for the actually cleaning and then the green felt cloth to make sure all of the cleaning fluid got off the iron. Worked like a charm!
I like to pin and in this case since the pants were pinstriped it was really easy to ensure that everything was straight.