Start at one heel and trim off the hoof wall to the toe, then stop and go to the other heel and trim to the toe. For a nice even trim, make each cut with the nippers only one-half the width of the nippers' cutting blades. With practice you should be able to trim from heel to heel and get an even cut. One of the biggest trimming errors is digging out the quarters, which happens when the person doing the trimming cuts the hoof wall too short at the quarters. Instead of following a straight line from heels to toe, the person follows the line of the sole.
Popular practice is to trim the heels to the widest part of the frog.
In general this practice is okay, but if you're not sure leave a little more heel. A rasp used correctly can be your best tool, but used wrong can be your worst tool. After trimming with the nippers, use the rasp to level the bottom of the hoof wall, remove burrs, and smooth and round the outside edge of the hoof wall. Pull the rasp with one hand and push with the other hand, using equal downward pressure from both hands. Do not move the rasp until you have it flat to the hoof and making contact with both sides of the hoof wall at the same time.