Here's how I've always done it -
Mark where you want the bottom of the hem to be and run a basting thread along that line. Cut fabric about 1" below line. Fold raw edge to line, then fold again, resulting in a double fold with raw edge encased. Basting thread should now be at bottom of the new leg. Press the heck out of the hem, until it's as flat as possible. Sew with a longer stitch, then remove basting thread.