To fit a bit correctly it needs to be on a bridle, with the noseband and throatlatch undone and the reins attached.

A New Bit: if you are fitting a new bit.
Put the Bit to be fitted onto the Bridle then hold it up to the side of the Horses face. Put top of the headpiece of the Bridle level with the top of the Horse's head and the mouthpiece of the Bit level with the corner of the mouth, adjust it at this stage so that the length is about right so that when you put it onto the Horses head the bit will only need to be adjusted by a hole or two not six!

The Height of the Bit: to check the height in the mouth, the bit should fit snugly into the corners of the lips, just wrinkling the corners without pulling the corners of the lips dramatically up. Then pull lightly pull down on the Cheeks of the Bit, the bit should not leave a gap of more than 1/8".

The Width of the Bit: to get the width right, stand in front of your horse and make sure the Bit is level and that there is an equal gap at each side of the Horse's face between the cheek of the Horse and the cheek of the Bit. With a jointed Bit you will have to straighten the Bit in the Horse's mouth by taking the cheek pieces in your hands and gently pulling the Bit straight in the mouth. You should just be able to place one finger sideways between the bit ring and the horses lip on each side of the mouth or approximately ¼". If the bit is too wide a joint could hang too low in the mouth and could interfere with the horse's front teeth or hang out of the side of the mouth, a Bit that is too wide will slide from side to side in the mouth. At the other end of the scale if the bit is too narrow the cheeks of the Bit will squash into the Horse's face this could result in rubbing and pinching.

Loose Ring cheeks: there should be 1/4" gap each side of the horse's face. If a Loose Ring Bit is too narrow, as the cheek slides through the hole in the mouthpiece there is a danger that the lips can be drawn into the gap and nipped.

In Hand Bits: Very lightweight Nylon Mullen or Straight Bar bits with small rings are designed to be used with In Hand bridles, because the bit is so light and the cheek piece so small it is very easy for the bit ring to be pulled into the Horses mouth or for the bit to turn over in the mouth if fitted to an ordinary riding Bridle. On an In Hand bridle the cheek piece secured by the noseband so the fitting is much more stable.

Cheeked and D Ring Snaffles: the upper part of the cheek must fit the face without rubbing the Horse. Cheeked snaffles should have to top part of the cheek secured to the Bridle cheek by a small leather keeper to help to keep the bit in the correct position in the mouth. The Fulmer Snaffle must have the cheek supported to stop the cheek pieces from hanging down loosely.

Pelham and Kimblewicks: a Pelham or Kimblewick with a sliding cheek has the potential to pinch the corners of the lips. There must be 1/4" gap on each side of the horse's face. Too wide a bit will move from side to side in the mouth and interfere with the action.

The Rugby Pelham: with its floating "bradoon" rings should always be used on a Double Bridle with the floating "Bradoon" rings secured onto the Bradoon Slip Head.

Double Bridle: the Bradoon should be fitted as a snaffle above not in front of the Weymouth. The Weymouth should lie just below the Bradoon. For the most comfortable fit, the Bradoon should be 1/4" wider than the Weymouth, i.e. a 5 1/2" Weymouth would go with a 5 3/4" Bradoon. Correctly a loose ring Bradoon should be used with a sliding cheek Weymouth and an Eggbut Bradoon is used with a fixed cheek Weymouth.
The chain's action on any bit that carries one should be felt when the cheek of the Weymouth is at about a 45% angle. If it is fitted too loosely the mouthpiece can roll too harshly down into the tongue. If the chain is fitted too tightly the Horse will never get any relief from the relentless pressure. The chain must be twisted so that it lies comfortably flat against the lower jaw and should always be secured by a Lipstrap.

The Bitless Bridle: or any combination Bridle that relies on nose pressure needs to be positioned high on the nose it must not interfere with the horses breathing so must lie on the nose bone above the nasal cartilage but not be fitted so high that it interferes and rubs the projecting cheek bone. The nosepiece should fit the face comfortably and snugly but not too tight. As the rein is used and the cheek turns the nosepiece must still have its action well above the Horses nostrils.