you simply pedal backwards.
first learn rollbacks. it's easier to learn sitting at first then you can stand or sit once you can do them.
so raise your seat a bit if it's super low.
sit on your seat and with your feet on the ground, push your self backwards. put your feet on the pedals and try to go as far as you can.
you can use a small hill to help yourself get backwards momentum if you like as well.
another way is to do a small endo and when your backwheel it's the ground, start pedaling backwards.
practice.practice.practice.