This a pain, but here's how you do it. First remove the cuff. If it's a turn up cuff, then release the stitches holding it in place. This includes the tacking at the seams, and the hem stitching. If...
You can use carpet tape available a the hardware store. Fold the hem up inside the leg to desired amount and tape between the fold and inside top of the hem. You can use a rolling pin or heavy book...