First: don’t start with building anatomy. If you fill in the shapes without a plan of your pose, it can look stiff and lifeless.
Do start with a gesture. Keep it loose, simple, and don’t worry about anatomy or proportion — you can fix that later. Your goal should be to make the pose clear and expressive.
Now you can start using smaller shapes to build form and refine proportion and anatomy. You don’t have to follow your gesture lines exactly, it’s perfectly fine to deviate! Since these are cartoons, you can also break anatomy for the sake of expressiveness — heck, one of the reasons the cartoon is so expressive is because the animators feel free to go off-model and exaggerate.
Once you get more used to drawing pony bodies, you’ll be able to eyeball the shapes and proportions better and won’t need all the breakdown shapes. e.g., I’ve drawn these leg shapes so many freaking times that I don’t really need much more than the gesture line. Use as many shapes and guides as you feel you need! Don’t hold yourself back because you think you should be using less.
Hopefully that helps, I’m sorry, I’m not good at explaining things! D: