I am experienced in training. I have trained many of my own horses. That is not the sort of advice I'm looking for. Someone who has trained horses for other people, what should I know? Do I need insurance? Should there be a contract? What is they aren't happy with what's being done? Etc, thank you guys!

Contract and insurance 100000%.

Make sure all communication is written- text messages, etc. That way if something goes sideways, you have proof written down. People are super sue-happy nowadays, and you have to protect yourself.

To avoid unhappiness- establish expectations from the onset. What do they want out of the horse, and what can you do to get the horse there? Explain to the owner that it may take more time than expected, as some people seem to think that they can pay for 30 days and get a good solid broke horse out of that. Sometimes they can! Sometimes horses are born broke. But sometimes it takes 30 days just to get the horse doing groundwork with a saddle on it's back, and you need to be clear that while you're working towards a goal, you can't guarentee anything in that timeframe.

My sister trains horses and she doesn't have insurance. She mainly trains for people we know and she has records of everything she does.

100% have contract and insurance. Also have video/pics of how horse arrives at your place and periodically throughout the stay in case anyone tries to come back on you.

Some things to make sure you consider or have nailed down:

Who's paying for feed/hay? Is owner supplying? Is it included in the training price?

What happens if horse is ready to go/doing everything owner wants ahead of schedule? Do you send home early? Conversely, what happens if horse is just lagging behind immensely?

What vet are you going to use if horse gets hurt? Does he have a current Coggins/vaccines/worming? What farrier will you use for him if he needs it during the training stay?

Are you going to let owner come out to visit during the training stay or is it more of a "drop your horse off and come back in 30 days please"? (I personally only do the second one, owners coming out multiple times has been a PITA when they expect Sparky to be dancing ballet four days into training and can't understand why he's still trying to kill you)

Make sure owner is fine with any special tack/equipment you use (I use Noavels in training as a hard rule and some people don't like that. That's okay, I don't need to train for them).

What are the expectations? If horse is a fire breathing dragon to start with and owner wants a kitten who can babysit kids after 30 days, it might be a good idea to lay out realistic expectations (and get it in writing!! CYA as a rule!)