For upgraded accounts with a training center you go to Estate -> Training Center and there will be a list of horses available to train with a green Train button

For non-upgraded accounts I think you can use it by going to Town -> Community Training then select a training center. A list of the horses available to train at that center should show along with the green Train button. I haven't used it this way since I upgraded but I think that's how it works.

Omg thank you so much!!! I seriously would've never found it without your help 😭❤️