Well, let's see.... Your horse tells you! You can follow what seems "right" to you. Develop a plan so that you introduce every part of your harness, learn to fit it correctly (but it doesn't hurt to also leave it loose and floppy for part of your training).
I have varied. If you've read any of my other posts, you know that I have often introduced bits on "hangers" or real headstalls as early as suckling and weanling ages to keep mouths busy and fast little minds working. You also may know that I've taken some heat from others who have felt that that's much too early and "bad" for the little ones I've done it with (not all of ours have been introduced to harness/bridles/bits as weanlings or yearlings!). But those ponies (and full size horses) have gone on to being driving and riding ponies and working, riding horses into their mid-20s, so for me this has worked. Other ponies/horses haven't been introduced to bits, harness or saddles until their mid-teens - and some of those were much harder to deal with.
In the past, our foals have often worn sheets and blankets. That introduces the feel of the surcingle, a breast plate and breeching (if it has the tie behind the haunches)! When introducing our foals to leading and being tied up, I use a belly or girth rope and/or a "butt rope". Again, that introduces "bits" of harness early yet you build on it from that time forward. I often add other parts - as weanlings if they've fit and they've accepted the parts. It takes very different amounts of time for different horses - I still have some that don't like working with open bridles and others who don't seem to like/accept blinders - after years of ground and hitched driving. When working a horse on the ground - lounging - I often added a rope around the hind quarters. It will help to teach a horse to driving up under himself with his hind quarters - a beginning step to collection AND a simple intro to loosely fitted breeching.
Voice commands - use what you want and start incorporating them when you groom your horse, when you lead your horse, when lounging and then finally when you ground drive and when hitched. The most important one is "whoa". Over the years, I've used different terms - some I've completely dropped from my vocabulary BUT if I have a horse that I know uses that word or word group, I use it too! Until just 4 years ago, when I started driving "draft style", I'd never used step or step up. Nor step over. Nor ez as a command to slow down the gait or shorten stride. For a lot of my original handling, I never used voice commands at all. I REALLY started using them when I started working with the smaller ponies that I put our precious daughters on. I wanted to KNOW that that pony would respond to MY voice regardless of little legs bouncing on their sides or butts a foot off of the saddle with each stride. Some ponies wouldn't accept that - they didn't become beginner riding ponies. There were ponies I didn't put my middle daughter on (she spent the least amount of time working with any "green mounts") and several more sensitive larger horses that my youngest daughter's more "busy" body never sat either.
As to bits, I started with what I had. The 2 first bits I had in the 70s and again in the mid-90s are simple snaffles w/ 2" rings & 4" mouth pieces- I used them on our first small shetland of 37" and on a 14 hh arab mare. They were slightly rounder mouthpieces w/ larger rings. They were inexpensive and I had to watch for the "plating" to wear off and make what was left sharp. The ponies and I worked with it. Later, as I found different bits I bought them when I could afford them and started playing - if needed. Most of those first bits were riding style bits (eggbutts, d-rings, full check snaffles). Some had larger diameter mouthpieces, some had smaller ones. One had a triangular mouthpiece (& one of our ponies LOVED it while I owned her) - very small diameter. Most of mine, I've found they like smaller diameter bits. Then I found copper and french link bits - in sizes that fit a lot of our ponies. YAY! Others told me to try different Myler style bits - the "real ones" are beyond my budget in most cases and the "knock offs" are too thick in diameter. The ones I borrowed and tried with my more experienced ponies didn't do well with any of the styles I tried... Not long ago, I had not one but TWO ponies that just weren't accepting any bits I owned or any that I borrowed. Teeth checked - no caps, no wolf teeth, no sharp edges, no scarring along the cheeks. I bought a mullen mouth bit, but it wasn't wide enough from ring to ring. SO - I braided up my own bits and the one worked for one (thicker haystring) and the other worked with the 2nd (thinner haystring). The one graduated to a regular driving snaffle now, the other never graduated out of the haystring, home made snaffle before I sold her.... after a 18 months of ground driving and hitched driving.
As you work with each horse, you will find that each is different. Even as you learn more - you will find that how you work with the same horse will change. The basics will remain the same - but you will develop more indepth understanding. You will develop a lighter "feel". You develop a "dance". It's not the same with every horse - training is not a cookie cutter project - they are all different and you keep doing different things if one thing doesn't get the response you want - until you and your horse are working as one, together.
Sorry, not a recipe for training. Even when I was in my teens, I found that not all training times nor training techniques worked for every horse OR even every rider... There isn't a timeline. AND yes, I DO HAVE/HAVE HAD a couple of ponies and/or horses that I was able to start in less then 30 days - very green, but happily trotting along while either under saddle or in harness. Then we've developed the rest of the program to bring the pony/horse to a higher degree of learning/moving. Others spent months in ground driving and lounging before ever being hitched or ridden and still may have shown a tendency to be "broncy" - in my younger years that was quite exhilarating. Not so much now ...
"It takes the time it takes..." and one of my favorite sayings - "Eat an elephant one bite at a time."
And I'm still learning!!!! I figure the day that I completely quit learning or accepting that there may be a different way to "skin a cat", that's the day my ashes better be spread.
And with all these "books" I write for posts, I should put them together into a real book!
I just got my horse trained to cart. Last fall i started her long driving with just her halter and a homemade surcingle I had made. . We went thru town. down highways and just anyplace there was activity. She did great so after about a dozen lessons that way..I put a bridle on her and did the same with a bit.. Maybe did that 2 or 3 times and then we took the winter off. A few weeks ago put a real harness on her and her driving gear including blinders. Spent about 15 miinutes 2 times long driving her. few days later hooked up some pvc poles and had her pull them 2 times for about 10 minutes. spent 10 minutes pulling the cart behind her a few days later and then hooked her up and off we went. She has been wonderful!
