How do you teach a horse long and low or to round their back? Need pointers and ideas

If you'll search "collection of horse " on YouTube, you'll find a lot of information. Your horse needs to be soft on the face , first (videos on this as well, on YouTube).

Classical dressage, not competitive dressage is a good place to learn how to do it correctly. However, many people don't want to take the time to teach is correctly. Try googling classical dressage training long and low.