New horsegreen and i need help

shes actually not mine, but shes my horse's neighbor, and they've been inseprable pasture buddies for a couple weeks now. i talked to her owner and she barely has any time to come out and ride her, and she asked me to train her for her. she was a rescue horse, and only knows the basics- walk, trot, canter. today was the first day i rode her. i got her to bend and wrap around my leg in the walk and trot, but the canter was a disaster. her correct lead feels like the wrong one, she bucks (big ones, not little kicks. they don't unseat me, but theyre annoying nonetheless. as soon as shes done, i keep her going.), will do sudden stops in the corners, and on the right, she slides over the left (i cant keep her on the rail on one side because there are cattle runs or whatever they are), despite my lifted right rein and left leg. i know its only her first ride in like 3 years, but how can i correct this soon?

Okay, it sounds like the horse hasn't been properly worked in a long time. If that's the case, you can't even think about the canter right now. You need to build up the horse's conditioning gradually and start working both her mind and her body. If you start with lots of trotting and cantering after a long lay-up or periods of little work, you're just asking for strained muscles and damaged tendons. Plus, when horses have that much time off and then start to be ridden again, they can get a little fresh. The bucking is probably a result of pent-up energy: she's playing. That feeling of unbalance at the canter is a result of poorly developed muscles: she's not physically capable of cantering correctly right now, so don't ask her to.