When fitting a bridle for the first time, assemble the bridle loosely and attach a bit.
The very first step is to adjust the height of the blinkers via the buckle at the top of the cheek, so that the eyes sit centrally in the blinkers, where the blinker is the most shaped, to give space to the eyes.
Secondly, adjust the height of the bit via the buckle at the bottom of the cheek, so the bit sits correctly in the corner of the lip. Then the winker stays can be shaped and adjusted to suit the conformation of the head. Form them into shape so the blinkers maintain the required clearance over the eyes whilst looking neat and tidy. The winker stays normally sit underneath the browband, and the length is adjustable via the small buckle at the top of the headpiece.
Thereafter, buckle up the noseband and throatlash. The noseband should be tight enough to prevent the cheeks from bulging outwards when there is contact on the bit via the reins, but comfortable enough to allow the horse to flex the jaw to facilitate swallowing and salivation. Two fingers clearance is a good guide. The flash noseband, if used, can be fitted with the same rules.
The throatlash should be tighter than on a riding bridle because it is vitally important that the bridle cannot be accidently pulled off over the ears. But it must also allow the horse room to swallow and flex the jaw even when there is flexion at the poll. If in doubt, push the nose inwards to check that the throatlash does not restrict the throat during poll flexion.
Check the browband particularly just behind the ears. Driving horses need to be able to swivel their ears comfortably in order to hear verbal instructions from the driver behind them. Therefore the browband should not cramp the ears or restrict them in any way. Horses with a wide forehead or those with thick forelocks sometimes need a browband one size up to allow extra room.