Personally, I like giving my horse their head, walking around on a loose rein for a couple circles each way. Then I'll ask for a bit of stretching through the neck and ask for a long and low frame for the trot before getting a nice "outline". I think they need to get in the mindset to work as well as getting physically warmed up. Once mine starts really stretching forward, looking for contact and moving from behind I know they're ready to start the warm up. I tend to go around the arena, doing 20m circles and maybe some figure eights as well, just to get them thinking. Usually it takes me 5-10 minutes to warm up, depending on the horse and how much time I can spend getting them ready.
How do you warm up? What are you looking for when you start your ride? How do you know your horse is warmed up?
Also- going to start to ride my ottb in the next couple weeks. The horses I've been riding all have developed toplines that they use already. My guy has zero muscle right now so we'll only be "working" for like 15-20 minutes at first. Just thought I'd ask, does anyone have any tips on how to get him using his back at the beginning of my ride?