If you're alone and don't have a quetz you should keep in mind that rexes get really easily stuck. Just get him/her to a nearby rock which is high enough and it usually works out, but make sure you keep a flyer close to you in case you need to reposition. if this doesn't work then you can try standing on a cliff and shooting down on him.
If you are on an animal it's way easy to tame these guys. I'd also recommend having the kibble and having a creature of any kind with high health. You can knock these guys out on foot too, but it's a lot harder and you risk other animals killing you/the rex. I'd also recommend not going after any mate boosted rex, since the torpor you do is directly related to the damage you do.
I have figured out a new way to tame dinosaurs cut off the dinosaurs meat wise in the area try and keep the dinosaur in the area and send a dodo bird out to attack the tea Rex with bunches of Botox in and it had the tea Rex eat the Botox and it’ll knock it out you go feed it meet make sure you have plenty of Botox in and you’ll have a pet your ex and no I would not recommend trying this whenever your level one side effects may be dead dodo bird hey might find you and run but seriously You better find a good hiding spot