With your left hand, take the bridle by the crownpiece and line the bit up with the horse’s lips and place it at his lips. Note, I said “place” Once it is there, leave it for a second or two. Do NOT try to jam or roughly pull the bit into the mouth. Though you MIGHT get it in that way the first time, from there it gets harder each time, not easier, eventually leading to a horse that realizes he can clamp his jaws and frustrate your efforts.
Instead of trying to force the bit in, reach a finger to the extreme corner of the mouth (where there are NO teeth) and tickle the gum. The horse will reflexively open its mouth, giving you the chance to apply the bridle. But, again, take care not to be SO excited he has opened up that you forcefully pull the bit in. Slowly let him accept it and he will soon do so willingly and happily.
The same principle applies to unbridling. Again place the halter on his neck, Unbuckle whatever is necessary to unbuckle to allow the horse to release the bit easily (curb chain, caveson, etc.).
Then, lift the crownpiece and take it off the horse one ear at a time and once it’s off both ears STOP for one second. This gives the horse warning, the bit is coming out. He can then relax his jaw and release the bit. This is VERY important! Let him release the bit. Pulling it out bangs the teeth painfully and leads to the horse not either taking the bit or letting go of it. A horse that resents being bridled and unbridled is a problem. A horse that willingly takes and releases the bridle however is a joy that leads to all the fun possibilities that come with riding.