First step, if you want to try a not entirely drive by approach, is to pinch the skin and give treats. I will practice neck and pecs (stand to the side). Hold off a vein and give treats. Then add a poking sensation. Needle cap. Pen. Toothpick. If you have nothing more failure proof to inject, you could in theory then progress to a sterile needle with nothing attached to it. You can start even with a super fine, short thing that won’t do a lot. I haven’t done that personally because I do still want to try to minimize the actual poking they have to endure, and also because after the prep work from the trusted person, it’s not been that big of a deal once it is time for injections. Especially when I’ve done the pecs, as the approach to the neck is usually the worst part of it all.
If you expect a fight, even when practicing, you’d want to position pony in the stall with the opposite side right up against a wall. With my big guy, we have to kind of shove him into the wall to keep him from trying to point any of his sharp ends at the vet or use his body or head as a defense. It’s way safer than on the cross ties or something. (I can inject that horse IM now on the cross ties, but I always ask the vet to inject, especially if IV, in the stall and then bring him out if he needs to be somewhere else for the exam). But practice on both sides. One may be easier for pony. One may be easier for you depending if you are right or left handed.
At some point, vet may have to just teach you appropriate technique and give you some drugs to try. If pony needs a lot or a mixture of things (i.e., not just Dorm), what you don’t want to have happen is trying it for real on the day of the appointment and then you don’t have enough sedation when the vet shows up, and then no one can get any more into him. I have a feeling that even if he responds pretty well to the twitch, you may still need to twitch + pre-sedate.
I am assuming you have tried xrays without sedation and that doesn’t work? I try not to sedate much for xrays if possible, especially if you are also blocking and trying to see the horse move after, potentially. I’m guessing that vet phobia is too much for him to stand for imaging without something?