There are several techniques for hemming pants. Some methods involve hand sewing, using the blind hem stitch or the slip stitch, but you can also hem pants on the sewing machine, which is faster than hand sewing.
If you have a pair of ready to wear pants that you would like to hem, you may want to cut off a few inches in the hem area in order to create the desired length. Make sure that you allow for the hem when doing this, or your pants may wind up being too short. The raw edge that is created when you cut off some of the fabric should be finished in some way. One of the easiest ways to finish this raw edge would be to serge finish, but if you don't have a serger, simply sewing an overcast or zigzag stitch would also work.
Of course, if you are sewing a pair of pants from a commercial sewing pattern, you will already know that the length of the pants are correct and you will also already know the width of the hem. The hem allowance is usually printed on the pattern piece itself and also included on the pattern sheet sewing instructions.
When you are ready to prepare the pants for hemming, use a seam gauge to ensure that the hem is even all the way around the pants leg. I like to use the seam gauge to measure the hem, and then press the hem up all the way around both pants legs. If I'm hand hemming, I like to place pins near the finished edge of the hem. This would work for hemming on the sewing machine as well, but you will have to pull the pins out as you sew.
There are two choices for hemming pants on the sewing machine. One choice is to use the machine built in blind hem stitch if your sewing machine has that particular stitch. The blind hem stitch usually involves changing the settings of the stitch width and possibly the stitch length as well, depending on the brand of sewing machine. Also, when using the blind hem stitch on the sewing machine, the fabric must be folded so that there is a small "lip" of fabric. The blind hem stitch is a more professional finish, but is often confusing at first. If you choose this method to hem pants, it would be wise to check your sewing machine manual and practice the technique on some scrap fabric first.
Another choice for hemming pants on the sewing machine is much easier and faster. This method would be used on more casual garments. After the hem has been pressed up, place the pants leg on the free arm of the sewing machine, then place the sewing machine needle all the way to the left and sew near the finished edge of the hem. Sew all the way around the pants leg, slightly overlapping where you started hemming in order to lock in the stitches. When both pants legs are hemmed, press well.
Hemming pants on the sewing machine is quite easy and fast, no matter which technique you choose. Take advantage of the free arm on the sewing machine because this makes it much easier to sew around each leg of the pants. If you remember to press well during the sewing construction, and press again after the hem has been sewn, you will have a professional looking pair of pants.