My first comment will be simple: If you need to ask this you are almost certainly not a good enough "horse person" to be dealing with a stallion. They're complicated and dangerous. Of course smaller stallions have a lower damage potential. Ever seen horse breeding where everyone has riding vests and helmets? There is a reason for that.
The easiest way to get a stallion used to sexual contact with people is to teach them to collect with an artificial vagina. It involves all the steps required and has a nice payoff at the end. Many stallions can be taught to collect on a phantom in a few sessions (a stimulus mare helps A LOT and some stallions won't work without one.) If you have access to a phantom and an AV that would 100% be my starting point. Some of the most important steps:
Allowing you to touch his belly and flank, then touch between the legs. Finally touching his penis while erect. Most stallions will not be fans of any of those things at first. Work from the top down and don't move forward until you and he are comfortable with the previous touching. Be ready for kicking or turning to bite. Stallions are very good at kicking sideways right where your knees will be. You should be doing this LONG before anything sexual happens.
Leading and controlling. You should be able to lead your stallion around anywhere. Next to mares, past other stallions, into buildings, everywhere. It shouldn't be a big fight with him dragging you around. Never get into a situation where you feel like your control is slipping away if you can help it. He needs to learn that when you have him in a halter he is following your lead and not the other way around.
Control while aroused. Once your stallion gets that mare scent or sees a nice horny mare you still need to be able to control him. You should always feel confident that you could stop and turn him around at any point. If something goes wrong he needs to trust you enough to stop and go where you want him to. That means you should think to yourself "I could get him off a mare he is trying to breed"
On the phantom and collecting. When you feel safe and in control its time to get him on the mare or phantom. Make it a happy and low stress scenario. If he wants to sniff and lick the phantom for 20 minutes go for it. Just keep his attention on it or try again later. Don't just hang around eating or grooming near the phantom. It is supposed to be a sexual experience he associates with sex. Mare scent or a tease mare help a ton. This is not a simple thing to teach and I'm certainly not an expert but you can find plenty of videos.
If you don't have access to a phantom or a mare I can't help much. Teaching a stallion to be sexual around people isn't too easy unless you start them young. Once they get the experience once though they are VERY fast learners. You could certainly let him breed a mare in hand in a certain place to get him into the concept. Then just move your mare a bit further away and have a conveniently placed human for him to mount. The first time is the hardest.
Being naked in the stall is fine but probably won't do much for him. Wearing cloths is probably a way weirder concept for horses. Just keep the hard toed shoes on. You do have hard toed shoes / boots right? Go slow and get some mare scent. Stallions require a very specific handjob to feel anything good. Its easy to annoy a stallion by fondling him aimlessly.
If you have more specific questions feel free to post them.
