Summarize:

Before you start trimming your horse's hooves, you should learn the parts of the hoof.  Learning the hoof anatomy will help you to know which areas of the hoof need to be trimmed, how they should look, and how best to trim them.  The outer hoof line and wall surround the exterior of the hoof. The coronary band connects the hoof to pastern, which is the part of the leg where the fur starts. The sole of the hoof is the entire area on the bottom of the hoof inside the outer hoof wall. The white line is just inside the outer hoof line. This area is very prone to hoof problems. The frog makes up a large portion of the hoof, starting at the heel and ending in a point at the center of hoof. After your horse's hoof has been fully cleaned and soaked, you will be able to assess which areas need trimming and how best to trim them.  Each hoof will be different every time you go to trim them.  Carefully look over your horse's hoof and decide your best course of action.  If you're unsure how much to trim, aim for trimming less rather than more so you don't injure your horse. If you trim off too much of the hoof, your horse will be very sore. Look for areas that are cracked that might need to be trimmed. Judge how long the the outer hoof wall is. Decide if the front of the hoof will need to be trimmed. See if the outside hoof wall is asymmetrical in any areas. Using the proper position when trimming your horse's hooves can help both you and your horse to be more comfortable during the process.  Holding your horse's hooves correctly can also allow you greater control over the hoof and let you get the hoof as level and well shaped as possible.  Always use the proper position when trimming your horse's hooves.  Hold the front leg by lifting it up and back, towards the horse's chest.  Rest the hoof on your thigh. Lift the back leg directly up and place your own leg in front of it.  Bend over and reach down to work with the hoof. When lifting the horse's leg up, always make sure it is bending with the natural motion of that joint. Don't try to lift the hoof if the horse isn't cooperating. To trim the hoof you will use your nippers, which are similar to large nail clippers for your horse's hooves.  The nippers are used to remove excess length in the outer hoof wall.  Using the nippers is a good way to quickly remove parts of the hoof, roughly shaping it before finely trimming the remaining areas.  Place the clippers on any long areas of the outer hoof wall. Clamp the clippers down to cut the long areas of the hoof wall away. Work slowly and make sure you are trimming to the length you need. Trim the front of the hoof at a 45 degree angle to avoid a sharp toe. As you clean and trim your horse's hooves, they may become unleveled or rough in some areas.  The rasp is a steel file that is used to correct any unleveled areas of your horse's hoof.  Dragging the rasp across the hoof will remove some of the hoof with the stroke.  Use your rasp to file down any leftover rough areas or spots that your nippers might be too large for, but make sure to use the rasp at a shallow angle so you don't injure your horse.  Don't rasp both heels at the same time as this can cause them to become uneven. Keep your strokes short at first until you gain more control and have more practice rasping. Keep your strokes as flat as possible in order to keep your horse's hoof flat. After you have leveled the outer wall of the horse's hoof, you will need to trim the sole of the hoof down until it is below the outer wall.  This will help keep pressure on the outer wall of the hoof rather than on the sensitive inner sole. Make sure you only trim dead flaky tissue. If the tissue feels elastic when you stretch it between your fingers, do not trim it since doing so could injure your horse. The outer wall of the hoof should be longer than the sole. After you have cleaned the hoof, trimmed it, and made sure it is level, you will want to look over the hoof one last time.  This will be a final check to make sure any problematic areas have been dealt with, that the hoof has been fully cleaned, and that your horse's step is level.  Check for symmetry in the hoof. Try to make the hoof even on all sides. The bottom of the horse's outer hoof wall should be flat.
Know the parts of the hoof. Determine how much needs to be trimmed. Get in position. Trim the hoof. Rasp the hoof with short, flat strokes. Trim the sole of the hoof. Check the hoof.