got it 
Dressage 101
Work the dressage training scale.
Get a good trainer to help you if you can.
Ok I guess it depends on whether you are talking about teaching a horse to step up from behind, lift its back and accept the bridle or whether you are asking how to achieve this in a horse who is already trained and fit but this is what I do to teach one. First of all I really don’t like the word frame as so many people create “fake” frames with gadgets.
A horse must understand the simplicity of the aids first. Go forward from the leg supplemented by cluck, whip or spur as needed. Whoa from the hand… sharper bit if needed. Once you have that then you go forward from the leg and ask the horse subtly with the hand to give and lower into a stretch. This is the beginning of the horse engaging its hind end and lifting its back. You need a lot of this as well as lateral work to strengthen the horse before you could expect a horse to carry itself in a “frame” with an engaged hind end, lifted back and light front end Some horses have the conformation and natural balance to do this sooner than others.
If you’ve never felt a horse do this before you will benefit from riding a schoolmaster with a trainer.
Have a nice soft contact and doing lots of circles and transitions. Bending and suppleing. Always remember to have soft hands and try to ride forward into the bridle. A true frame is hard to achieve at first and you’ll have to practice and build those muscles. Some horses have better build than others so don’t be discouraged if it takes time. Try to keep it simple. Having a trainer helps because they know the correct timing but there’s lots of helpful dressage books and videos with great information.
Good luck !
