The back lever is a gymnastic strength move, it requires a lot of upper body strength and basic gymnastic conditioning before you even attempt it.
This exercise is dangerous for many people, use caution!
The exercise can be done on still rings, the high bar or a chin bar set a fair way from the ceiling. You hang upside down with an underhand grip. If you're using a bar, the bar has to be behind you so try hanging with the bar in front of you and walk you legs through.
When you have the position, lower yourself, pivoting at your shoulders until your body is parallel to the ground (or as close as you can safely get) belly facing downwards and hold the position for several seconds. When you can't hold it anymore bring your self back up to vertical.
Take care as you have to be able to get out of any situation you get into, so don't go too low on the first try and make sure you only do it over a crash mat or with a couple of helpers to catch you if you have to let go.
If you're confused about the description, the HTML version of this FAQ available via the World Wide Web, contains pictures which will be below if you're using a graphical browser like Mosaic.
Many thanks go to Keith Smith for patiently explaining the back lever to me.