You will need some tools - Measuring tape or ruler, iron, chalk pencil, seam ripper, fabric scissors, fray check liquid adhesive, dressmaker's pins, needle, thread and possibly a thimble especially if the pant fabric is thick like denim or corduroy. 

Preparation - If possible, try to practice on a pair of pants that are inexpensive or with which you can work out mistakes if they occur. Try on the pants wearing the shoes with which you plan to wear the pants.  

 1. Measure - Measure the amount you would like to take up.  (The best way to do this is to ask a friend to help you, but pinning up the amount you think will work, putting on the pants and then checking the fit at the floor in a full length mirror is an alternative. You may have to take the pants off and put them back on several times to get the length just right if you are doing the self-measure method.)

2. Marking the hem - In general, pants should fall at the arch of the foot in the front with enough length so that the fabric "breaks" or indents in a small fold at the ankle. The back of the pant leg should be 1/4" longer than the front of the pant leg.  The back of the pant leg should cover the heel of the shoe but not the sole and it should not drag the floor, unless that is the length you would like the pants to be. The depth of the hem (the amount of fabric turned up)  should be about 1- 1/2" to 2". (If you are hemming new jeans or 100% cotton pants, try washing the pants first before you hem.   (If the amount needing hemming is small, they may shrink enough to fit well without taking any of the length up.)

On the outside or "right side" of the pants, mark the line for turn up with your chalk pencil. Make another chalk line 1-1/2" to 2" from the chalk line toward the bottom of the pants. This second line is the allowance you will need to turn up for a hem in the pant and this last chalk line will be the line on which to cut off the pants or if the original hem falls in this area go to step 3. 

3. Remove original hem -Using your seam ripper, remove the previous hem. (If the amount you wish to take up, plus the width of the depth of the hem falls above the original pant hem, you can just cut off the pant above the  original hem and skip ripping out the original hem.) After taking out the original hem, draw a  chalk line on which you wish to cut the pants off a distance of 1-1/2" to 2" below the turn up line.

4. Cut - Using your fabric scissors, cut off the pant at the bottom chalk line.  Save the pieces that you cut off. They pieces can come in very handy at some later point if you wish to mend a rip or tear, make an applique, or if residual shrinkage makes the pant too short after several washings.

5. Press -With the iron, press the old hem area flat with the appropriate setting on your iron for the fabric of the pant. Use steam for cottons.  If your chalk mark for the turn up disappears during pressing, measure up from your cut line bottom the amount that you added for your hem and remark the turn up line.  Turn the pants up along the turn-up line and press with the iron. 

6. Edgefinish - You can whipstitch over the raw cut edge of the fabric to prevent the hem edge from raveling or alternately, apply some fray check liquid adhesive to the cut edge and allow to dry before completing the hem. (Jeans are usually turned up 3/4" and then turned up 3/4" again - enclosing the raw edge of the cut bottom in the turn up. Stitch by hand or use a sewing machine. This type of hem prevents raveling without fray check or overcasting. You can do the same with other pants besides jeans, but sometimes this makes the hem too bulky and the pants won't hang well. If this is the case, "edge finish" the cut edge by whipstitching over the raw edge of the fabric with your needle and thread or using fray check. )

6. Stitch - Using the needle and thread, slip stitch or blind stitch the hem in place.

7. Since the pant leg is a cone, pants that are not straight leg pants will either have too much room or too little room as you hem the fabric up inside the pants. You may need to take small tucks on the inside if there is too much fabric (bootcut pants) or make a short cut (peg leg pants) in the hemming area to allow the hem fabric to spread out so that the fabric does not pucker or pull on the right side. If you do make small cuts in the inside hem area to allow the fabric to spread out, finish the edges of the cut with either stitching or fray check so you will not have threads raveling out to catch your shoes or toes.

8. Final press and enjoy your pants now at the perfect length for you.