Put the pants on inside out, pin the crotch to the desired length. You should leave approximately a half inch of space between the leg and pin. Now run the pants through a sewing machine on the outer side of the pins starting on the edge of the original hem.
Starting at the mid-thigh of the inner seam, use a sewing machine to stitch a new inseam. Stitch upward toward the crotch, gradually making a wider seam in the crotch area, then tapering off down the other side.