First whoever is going to be wearing the pants needs to try them on for you. Then you put a pin in the back of the pant leg where they want to hem to fall. This is to mark the finished edge of the new length. You only need to mark one pant leg. Carefully match seams, lay the pants flat and add 2 inches from the pin. Cut off extra fabric. You can add less but I like to have a wide hem in case they need to be re-adjusted in the future.
Next you are going to zigzag stitch around the edge of both cuffs. I like my stitch length set to 2 inches and my stitch width set to 4 inches for this. The needle should go into the fabric on one side and then barely into or just over the edge on the other side of the zigzag.
Turn pants inside out; fold and pin a 2 inch hem line. I use 1 pin in each seam, and 1 pin across from each seam. Then one in between each of those pins to hold it in place and keep it straight. Keeping the pins partially pulled up and away from the fabric so you don't melt the heads iron the fold.
Baste hem 1/4 inch from your zigzag line. For basting set your stitch length as long as your machine allows because you will be pulling this seam out at the end. My machine goes to 5 inches stitch length. CAUTION: Do Not snip the thread ends, and don't reverse sew or lock your thread in. This will make it difficult to remove when the time comes. Remove pins.
Switch to a blind hem foot on your machine and the blind hem stitch. Mine is number 7 on my machine. I took a photo so you could see which stitch I am talking about. This foot doesn't allow reverse stitching so sew past your starting point a little ways to lock it in at the end.
Fold the hem inside the pants so only your little zigzag edge is sticking out. Start your sewing close to one of the pants seams. Keep the seam guide on the foot right in the fold of your baste line.
Once you are done stitching turn your pants right-side out again. Remove baste stitching by pulling on one of the two threads. If it breaks don't worry find the new end and keep going until it is completely removed. Some fabrics can be pretty stubborn about this part. If it's really difficult you can use your seam ripper from the right side of the pants so you don't chance cutting your other stitching.
TaDa! You are finished. Notice that the stitches should be barely visible on the right-side of the fabric.