Well, I don't do it to make a living so I go slow.
-Usually start with sacking out using various floppy things like shirts, coats, plastic sacks, tarps etc.
-I put my weight on them in various ways so they get somewhat used to adjusting their balance. That's one of the most interesting sensations, a young horse trying to figure out how to walk with a rider on their back for the first time or two. I'll work up to the point I can put most my weight on them without reaction.
-Next comes a junk saddle on their back, don't want to ruin a good one from a blowup (haven't had a blowup yet but you never know). Once they are ok with the saddle just sitting there I'll slowly cinch it up until they get used to that.
-Then it's driving time. I wont crawl in the saddle until they understand go, whoa, left and right. Use a couple lunge lines so I can hang out well behind the rear end then have someone lead them as I drive from behind. Once I can drive without a helper they are ready for their first ride.
-First time in the saddle I'll have a helper hold them while I jump up and down beside them. Then it's put a foot in the saddle and more jumping. That's good? Then it's time for my rump to hit the saddle. I don't give them time to think about what just happened and have my helper take off once I'm situated leading them. We then repeat the go, whoa, left and right exercises with the helper leading. Once that is down pat the helper fades away and riding has finally started.
Like I said, I'm not making a living at it but enjoy raising and training my own horses. I know most trainers can be on their back in just a day or two but I take my time. This whole thing starts when I get them home and takes until they are actually old enough to crawl on their backs. Fastest time frame will be a couple of months even if already old enough to jump on up. I'm going to minimize my risk of getting hurt so I don't progress to the next step until sure the chance of getting hurt is small.
Youngest I've ever started working on a foal was 6mos (sacking out step). By the time he was old enough to ride he acted like an old pro.