My daughter has decided that she is coming out of retirement...lol.  After about a 3 year break from barrel racing to focus on a few other things she wants to get back at it.  What's are the best ways to get her horse legged back up.  She also has been off for 3 years.  We want to do things the safest way to avoid any injury.  Any help is appreciated.  I was thinking a lot of trotting for a few weeks at least.  TIA.

I'm in the same boat- horse and I have also been off for three years!
I'm thinking a week of lunging, just to get her brain turned on, then increasingly long walk/trot rides. I'm hoping to be long trotting a few miles in the next couple months.
Personally, I started working out too, just to get my quarantine flab off and wake up my muscles.

Tell your daughter to get a good playlist going for the long trotting, and keep some kind of journal that she can document how long she rode/lunged whatever so she can gradually increase it.

ETA: I'm also adding ulcer prevention to my mare's diet to protect her belly as she gets back to work.

6-8 Weeks of slow process conditioning! Start with just walking and trotting for a few weeks, then go up to loping for another couple of weeks, then start doing some drill work to build the muscles that haven't been used in awhile. Then on week 7-8 I would start slowly looking at the pattern again. Horses can maintain their condition a lot better than a lot of people think, so make sure to give the horse days off to repair the muscle fibers that have ripped and started to fray (that's how they get stronger). A day off here and there matters more than repeated hard conditioning when bringing a horse back :) Good luck and tell her to just have fun! Take before and after pictures! It's a good pick me up when you start to think twice about the conditioning process!

Do it slowly. I take the winter off (beginning of december-end of Feb)  and it takes me 2 months to leg them back up enought to where I feel comfortable making a run. I start with a week of walking, stretching them, getting their topline used to the weight of a rider again without the stress of faster paces. Work on your circles, brush up on their buttons, backing, stops, etc. Then slowly add in trotting, just an easy working trot. I don't long trot until week 3. When I start long trotting I start for 2 minutes each directions & work up from there. Week 5 is when loping starts. Starting at 1 minute and working up. By the end of the 2 months I am doing 5 minutes of long trotting each way and 4 minute loping each way. At that point I feel they're decently ready. Eventually throughout the season I build up to 20 minutes of long trotting, 10 each way. And then 10-15 of loping, breaking it into even times each way. I ride mine 5 days a week, sometimes 6, but the 6th day is always a light day. Slow to start will be your best friend and the easiest on the tendons, which is more important than the muscles when starting up. A little muscle strain/discomfort is way easier to deal with than strained or torn tendons.