Start with tires, NOT a real drag. Tires are easy to move, don't hurt things much that they might bang into. A pickup truck tire to start, NOT a big tractor or heavy equipment tire! Lengthen out the traces, so singletree won't bang her in the legs, even if she gets going fast.
We do our dragging training with horses on LONG long-lines going in circles around the handler. Handler has a whip with lash long enough to reach horse WAYY out there. Whip is useless if lash won't reach horse, so maybe tying extra cord on will get you the length you need. Whip is the "reach out and touch" device, NOT a whacking tool to scare her with. Just a touch with the lash end is as much as most horses need, to encourage the forward and NOT IGNORE the handler, or voice commands. You can see horse is doing as asked from the side, no HANGING on their face, because most folks can't walk fast enough to stay off their face. But handler BEHIND horse can't tell they are being boat anchor on the reins. With no mouth release, WHY should horse give to the bit? Never gets any pressure release for reward! So horse will GLADLY pull your arms off, just like you TAUGHT HER to. We practice horse being reversed while out there on the lines, no helper to turn her around or you stopping her to manually reverse her direction. She has to turn as directed when you go in a vehicle, so this is her starting to learn how.
You will want to do this first couple dragging sessions inside an indoor arena, ALL doors closed, if possible. Safer for her so she can't go over or thru a wood/wire fence if you can't hold her in a spook. With an indoor she can't leave if she pulls loose. We are doing long lines on our horses, when they start dragging thing, so lines are down along their sides, not up along the spine. Horses can't turn UNDER the lines when they are low. You have better control of both front and rear of the animal, to keep forward going or contained.
You will want the helper person just walking along, holding a long rope thru the BIG, open, singletree clevis that will drop dragging tire off quickly if helper lets go of rope end. Then you quietly stop horse, put the tire back on, walk some more. Having tire able to be released FAST, keeps it from "chasing" her, into a full out brain freeze. Tire on, tire off IF NEEDED, helps practice her stops, standing quiet and walking calmly off too! You can advance to tire firmly attached as she stays accepting.
As she gets dependable with the tire, you can hook on a second one, rather than getting a bigger tire. You just want the resistance of drag, not huge weights that might scare her. She is a big girl, but horses need to be taught how to manage weight, learn SLOWLY how to be brave with their increasing load. You can't let her fail or start to fight, to get the load moving. She will get scared if something won't let her move it!! She is still getting a good workout, building strength with light tires. Tires swinging out on her circles give traces a lot of pull, changes the feel of harness, rubbing on her, though she STILL is expected to be obedient and responsive to commands of the various walks and trots. Do practice a calm walk, walk with a bit more forward, and a really MARCHING along walk as fast as she can manage without any trot to it. Have a command for EACH ONE, like Walk, Walk ON, MARCH, so she knows what you mean. Same with trots. She should be able to come up with 3 distinct trots, and get better in them as you practice doing them. Gives YOU power in your "gear shifting" for later vehicle driving, since you BOTH are comfortable with all these gaits because they are familiar to her.
When she is good on the tires, you can do some fake shafts.
She CAN NOT back up with fake shafts or travois!! Do NOT ASK her to try, you will CREATE problems with her!! No problem if you never ask her to back while wearing these training aids.
I use sapling trees, about 12-14ft long. I don't like PVC pipe, with an echo near her ears, can be broken with being stepped on. The saplings are bendy for dragging, stepping on, no extra noises, FREE. Those are tied on lightly to the harness saddle with VERY breakable string, one side at a time. Short session of one side, then do the other side with a sapling going the other way. Enough for a first day. You can work into longer sessions over a few days, with only one sapling at a time. Work up to all the gaits, with only one sapling on, watch that she stays nice for you. You continue the works as she stays quiet and obedient, until finally working with sapling shafts on both sides. Then again work in both directions. I never push or recommend it all is done the same day!
If the string breaks, you stop quietly, tie it back on. NO BIG REACTION from you, helps her accept shaft falling, being replaced, part of what happens! You probably want to get some better string now for the saplings. After knowing she is comfortable with both sides on, going both ways, get another sapling to tie across on the dragging ends, for a proper Travois. You DO NOT want the parts falling off here, since they are all tied together, wouldn't fall off evenly! This crosspiece added, gives everything a different feel, stiffens up the way the saplings move on her sides. Again, do enough work with this setup to know she is quite comfortable in it, listening to you. Then you could add the dragging one tire again from the travois crossbar. Get her comfortable with that too.
This is not going to happen fast, even if you can work her daily. You want her getting BORED with the additions, responding quickly the FIRST time you ask for trot or downward transition to slower gaits. FAST stop when WHOA is asked. Work up to better responses. She is just big, not DEAF or stupid. Drafts should be as responsive as a saddle horse size animal, to keep things safe. Accept no excuses, touch her up with the whip gently on first command, to show you mean TROT NOW. Usually makes them move better. If you wait to touch her up with a second command, you are TRAINING her to wait for second command! She gains confidence in KNOWING what you ask won't hurt or scare her, so YOU are reliable as a director of the driving stuff! She needs to have that confidence in YOU to be comfortable in her work, willing to try for you. Even horses who never had problems, take time and work to reach this stage, just no shortcuts to it.
She needs to be VERY GOOD at all these things, before you can think of putting her in a vehicle. These are her BASICS, from which you build all the other stuff so she can be the enjoyable driving animal in the future, in various situations. She needs to be rock-solid here at the beginning, every step, to allow reliable advances in her driving skills.