jump straight up. tuck knees to chest hard. land on feet.
hardest part of back flips is nadding up to try it the first time. mechanically they're really easy.
i learned by jumping off the grass edge into a sand volleyball court. first someone held my hips and i jumped up and tucked basically against his shoulder. then he moved to the side and spotted me. took some shit talking to get me to do it, but once i had done the first one, the rest were not bad.