This page covers how to get started in Headdown including prerequisites and solos that you need to make.
It's not safe to learn headdown if you can't sitfly. Headdown is inherently zoomy, and you need to be able to go to your feet to get things back under control without drastically changing the fall rate.
How do you know when you're ready? Here's a good litmus test. Do a trade with a freeflier who can lay a good base for you. Try the old Olav aerial dolphin test on your feet using your coach as the spaceball. That is, dock him/her, do a 360 degree turn, dock, do a flip, dock, do a cartwheel, dock. If you can do that dive, then you are a good enough sitflyer to try headdown.
You will be zooming forwards or backwards. You do not want to open up near other groups, so face 90 degrees off of jump run.
You must work your way through the following solos before you can fly headdown with anybody else. The only exception is if you're struggling with the solos, and you don't know why, then you get a badass freeflier to come take you out on your head with grips. Your natural ability will dictate how many solos it will take you to accomplish these dives, but figure on at least 10.
Be ready to do a boatload of these.
For when you're basically headdown but zooming.
This is the same as the one to be done with sitfly solos
The solos are intended to get you to where you can basically make something resembling an attempt at headdown. After that, you'll be just barely safe enough to Tahiko trade with somebody who's already a good headdowner, but you're still going to be kind of zoomy. Be quick to go to your feet when you zoom. Just get under control and try again.