Fundamental Principles
It should be accepted as a guiding principle that in a natural healthy foot, the outside rim of the hoof wall and the small portion of the sole directly attached to it where the shoe will rest are the only portions of the hoof to be worked on when preparing the hoof for the shoe.  The appropriate trimming should be done with nippers and a rasp.  The feet must be trimmed along the same plane.   The cannot be over emphasized, as the smallest deviation from a perfectly level hoof can have severe consequences, not only on the hoof, but also on the entire limb.

Securing the Levels
In the hoof itself, when the weight is borne unevenly, the lowest part of the leg receives an undue share of the load.  The pressure of the weight retards the growth and the free movement of the limb.  The hoof then becomes weakened, distorted, and deformed.   The limb, deflected as it is by an uneven load, takes the brunt of the pressure.  The bones and tendons mutually suffer from the strain.  The hoof wall must be made to be perfectly level, that is no more is to be taken from one side than the other.  This is determined by exact measurements taken at opposite points all around the hoof from the coronet band to the ground surface, with all having the same degree.

Balance
The hoof must be balanced to mechanical harmony.  In other words, from a line drawn through the longer axis of the frog the measurements to opposite points should be the same on both sides of the hoof.  This means that a longitudinal line through the center of the frog must at all points be the center of the foot or divide it into exact halves.  In this way only will the force of the footfall be carried through the bony column and be equally and naturally distributed upon the supporting apparatus of the horse’s bones and tendons and the weight bearing portions of the hoof.

Preparing the Hoof
In leveling and balancing the hoof, my practice is to remove only the portions of the hoof that is naturally about to be cast off.  That is, those portions of the hoof that are exfoliating and devoid of the natural moisture and flexibility that exist in healthy growth.  I trim the hoof wall to a level with the untouched healthy sole.  I aim to have every part of the wall, from the angle of the heels to the toe, receive a broad equal bearing on the shoe, unless there is some special reason for doing otherwise.  With some feet, having a strong healthy growth, this means a good deal of trimming, especially if the shoes have been on for some time.  On the other hand, there are feet that require very little trimming; therefore the work must be done with constant and intelligent reference to the inclination of the pasterns.   Sometimes the hoof will grow all to the toe, other times the growth will be located more toward the heel.  Or the hoof may be run over by having one side too high or the toe worn off excessively.  To rectify this, and to better the adjustment, a good idea is to look at the old shoe and see where it has been worn most and be partly guided by this.  The overall point is to remove the surplus growth from the hoof wall, or as much as it takes to bring the hoof back to its natural shape and alignment.  All of this must depend upon the judgment, expertise, and ingenuity of the farrier on the job.  The farrier should never lose sight of the fact that if the heels are allowed to grow too high, an excessive strain is put on the skeletal structure of the legs.  If the toes are too long the tendons suffer in a like manner.  The sharp lower rim of the hoof wall should be rounded off, and all stubs or nails should be removed.