Diamond Heist
Where to start? I'm assuming that you have limited experience with horses; or are you going to be one of those people who ask for advice (appearing to be totally novice) then, when it is given, turn out to be multi-talented and leave us all wondering why you asked in the first place?
How much have you done with your youngster, is it handled at all, seen any tack............?
Ideally you will have been doing lots of in-hand work with your horse; getting him used to you being his leader and he will be comfortable in second place in your herd of two, and looking to you for guidance if he feels lost or nervous of new things/situations. If not, then you really need to do this before introducing him to tack.
I start all youngsters with a Fulmer (full cheek) French link snaffle with play keys. There are several makes on the market. They must be fitted with Fulmer loops to keep them in the correct position in the horse's mouth.
If your horse has not done anything at all, I would recommend starting him in a lunge cavesson and doing some very short spells (five to ten minutes) of in-hand work, and light (walking only) lungeing so that you can assess his willingness to work away from you, and teach him that he must respond if you ask him to work. Have the lunge rope attached to the middle ring on the cavesson for best control. Work him like this for ten minutes a couple of times a day until you are both happy with it.
Only when he is doing as you ask should you ask for more.
Next I would introduce him to the bit, either fitted to the lunge cavesson, or by fitting him with a bridle. Once the bit is placed correctly in his mouth, you should send him away again quietly on the lunge, so that he can get used to the bit in his mouth.
It helps, when introducing something strange to a young horse, if you ask him to do something he is familiar with as a means of taking his mind off the strange thing, and helping him come to terms with it.
You should not do too much lungeing with a horse of his age/stage of learning. Rather do a bit of leading about in hand as a way of allowing him to safely get used to bits of tack, and only send him away on the lunge if he gets upset and you are concerned for your safety. Wear safety gear (hat and gloves at least; toe and body protectors too, if you have them).
If you let him see everything as you introduce him to it, and be quietly confident in your handling of him, you should progress together without problems.
If you have a horse who is confident in you and your handling, you should be able to introduce a saddle by first letting him see and inspect it, initially on the ground, then while you hold it. When he has had a look of it and begins to lose interest, you can then, whilst holding him on a slack rope, ask him to accept it against his chest and neck, before laying it over his back for a few seconds, then taking it off and holding it for a few seconds, before asking him to accept it again. If he is not able to allow it on his back, be prepared to settle for a compromise, such as going back to touching his chest/shoulder with it before trying his back again. Using that method (advance and retreat) throughout his training, you should be able to progress to asking him to accept the saddle.
If he will not allow something, ask him to do something he is already familiar with and then stop there, give him lots of praise, a bite to eat, and put him away to give him a break from any tension he is feeling.
Keep these sessions to a maximum of twenty minutes for the first couple of weeks, assessing him each time to see how he is progressing, so that you have an idea where to go from there. As he becomes more confident so will you and so will he...............
It's a bit of a minefield trying to advise anyone on these matters on a forum.
It's not so much what to do as what not to do, and how the handler deals with things when they go wrong. For some people/horses introducing a bit/saddle may be a very simple exercise, whereas for others............
As Cheeky has said, you could do with some help from someone with the experience you may lack, even now and then, to keep you right.
Good luck, and never be afraid to ask for advice and help, for your horse's sake as well as your own.