A segment from Matt Wilhelm's "Ground Rules" video: how to do a fire hydrant.
A few other points to consider:
- The more momentum you put into spinning your body around the head tube, the easier it will be to get your frame to whip around after you.
- Matt mentions that you need to lean forward at a certain point while rolling and whipping the frame around. It's scary, but necessary. This is the part that most commonly gets in the way of a rider learning the trick.
- Don't hit your front brake until your frame comes at least in line with the crossbar of your handlebars. You might be tempted to hit the brake sooner in the interest of preserving your face, but doing so will throw you off.
- As with any other trick: in the beginning, you'll only be able to land it when everything goes perfectly. But over time - as you get better and better at it - you'll be able to "save" the trick, even when the frame doesn't whip around quite perfectly.
Good luck!
Once you master the fire hydrant, you can use it to link to any number of other tricks; one example being Matt linking it to a decade in the video above. Here's another example, linking into a squeaker: