Well, there are two options here: first classic answer is “you don’t, you put your hand in the mouth and it’s gonna open it, period!” The second option is slightly different and consists in teaching different steps to your horse to make him want the bit and the bridle: lower the head, tip it inside, open the mouth, take the bit and let the horseman or horsewoman put the headstall on.
Your horse is your partner, not a beast whom you must fight all the time. So, when you come with the bridle, your partner should anticipate what you want and do the necessary to help you. The idea is to lower the head and to tip it toward you.
First, put your fingers right between the ears (if your horse is too tall for you, put your fingers as high as you can along the neck) and press. Just keep pressure on the same spot until the horse lowers the head. At first, lowering the head means giving up to the pressure by one or two millimeters maybe.
Good! That’s enough to release pressure and to reward: rub it with all your heart.
Repeat the process several times, you will see how fast a horse can learn to lower its head on demand. Pretty soon, a light pressure between the ears or on the neck will be enough to lower the head to the good level (roughly right below your armpit, we do not want to reach the grass when bridling, do we?).
Second, wrap your arm around your horse’s neck, same as if you were hugging it. Put your right hand on the side of its head and push. Same as before, just keep pushing until your horse gives up and point its head to you, one or two millimeter is enough at the beginning. Pretty soon, the feel of your hand on the side of its head will make your horse tip its head toward you.
Remember that timing is the most important parameter here: release pressure too soon and you teach your horse to resist you, release too late and you fail to reward your horse: then what’s the point to lower the head or to tip inside?
Taking the bit
When preparing a horse to take the bit, it is good to use the leading rope _ it’s smoother and you will not hurt your horse when training it and yourself.
You should wrap the rope around your horse’s neck and take the loose end in front of its mouth. Present the rope to its lips and wait. Leave some time to your horse to process that new cue. If you do not have the expected response, then you should put your thumb in the mouth and shake gently your horse’s head: we want to make it uncomfortable, not to hurt it.
As soon as the horse opens the mouth, release all pressure and rub it. Pretty soon, you will not need to shake the head anymore, then the thumb will not be necessary neither. Finally, presenting the rope to your horse’s lips will be enough to make it open the mouth, then pull gently the rope inside the mouth and let your horse chew it a little bit: it is a new feeling and it needs to get used to it. After a few moments, let the horse spit the rope out, do not pull on it as you should be careful not to hurt the teeth, the tongue or the lips in the process.
These foals and young 2 to 4 years old horses still lose their baby teeth and it can be pretty easy to hurt their mouth by pushing, pulling or hitting one of these weak teeth.