To measure an inseam for a pair of trousers, have the person being measured stand up straight, hold the end of a tape measure at his crotch, extend the tape down to the ground and read the measurement where you wish the bottom of the pant leg to extend. To get an accurate measurement, the person being measured should be in his underwear or in a pair of tight pants.
If the person being measured owns a pair of pants that fit well, you can take an inseam measurement from his existing trousers. Lay the pants on a flat surface, place the end of the tape measure at the point in the crotch where the two inseams meet, hold the pant leg straight and extend the tape measure to the bottom of the pant leg.
To hem a pair of pants, the pant legs need to be longer than the inseam measurement. Measure the desired inseam length, and insert a pin perpendicular to the inseam to mark the measurement. To create a 1.5 inch hem, cut off the pant leg 2 inches beyond the pin. Turn in 0.5 inch of fabric at the bottom of the leg, and then turn in an additional 1.5 inches of fabric. Hold the fabric in place with a few pins, and sew around the pant leg at the top of the hem.