Put on the trousers and mark the desired hem length with pins or chalk
Measure both legs to make sure the hem is even
Remove the trousers and turn them inside out
Fold the fabric up to the marked length
Press the fold with an iron to create a sharp crease
Fold the raw edge under again to hide it
Pin the hem in place all the way around
Thread the sewing machine with matching thread
Set the machine to a straight stitch
Sew close to the inner folded edge of the hem
Keep the stitching even as you sew around each leg
Backstitch at the start and end to secure the seam
Trim any loose threads
Press the hem again with an iron
Turn the trousers right side out and check the length