Pin all the way around the leg. Measure the hem you have just turned and then divide that number in half. For the purposes of this explanation, let’s imagine the jeans need to be turned up 3 inches; 1½ inches is, therefore, the measurement to use. The reason you cut the measurement in half is because you will be folding up the hem in half and actually hemming the full 3 inches.
Unpin the turned-up hem and then adjust it until it measures 1½ inches from the edge of the original hem to the fold; pin. Use a zipper foot for your sewing machine if you have it—this allows you to stitch close to the original hem. If not, adjust the sewing-machine needle as far to the left as is possible. Make sure that you are using an appropriate foot—if you do not, the needle will come down on the foot itself and break. Stitch all the way around the leg bottom, stitching close to the edge of the original hem and being careful not to sew the leg opening together.
Try the jeans on again to make sure you are happy with the length. Turn them right side out and tuck the hemmed area upward. The stitching line around the new hem will form a seam on the right side directly above the original hem’s stitching; it will be virtually invisible. Press the new hem flat.
To keep the French hem from showing you can trim and leave raw edge as is, ironing it up or down after each washing. Alternatively, use some fusible hemming tape and fuse the hem upward on the inside.