In the ranking of
strength skills on rings, on a scale from A to E, the back lever sits at the
A level. This means it is one of the easiest strength skills to perform.
That being said, it should also be one of the first skills you master.
Especially if you are working towards higher level skills like the front
lever (B level) and planche (C level). The back lever builds upper body
strength and teaches the important skills of total body tension and
coordination.
Alright, square one . . .

First, you'll grab the bar (or rings), get into a tuck, and invert yourself.
Simple enough.

Continue bringing your feet over your head until you can get in the position
shown above. For those who never did this as a kid, this exercise is called
"skin the cat".
You'll want to then bring your feet and legs back up over your head to get
back to the start. When you first start this exercise, you will want to stay
tucked as you bring your legs up and down. As you get stronger, work on
keeping your legs straight and moving your piked body back and forth.

Soon you'll get strong enough to start trying some positions. Whichever
position you choose to work on, I find it helpful to straighten my body as
soon as possible. On the rings, this is simply achieved by getting into an
inverted straight hang. With a pullup bar, you have a bar and doorframe to
contend with, so simply straighten like in the picture above.

If you feel ready, try lowering down into a one-legged back lever. Having
one leg tucked-in will take some of the weight off the skill. This position
feels a bit odd in comparison to a regular back lever, but it is easier.

If the one-legger gets too easy, just straighten yourself out and lower down
into a back lever. I find it easiest when I look at my feet as I lower down,
so I can see when I'm horizontal. After you level out, just pick up your
head and look forward.
In performing any of these exercises or skills, make sure you land
on your feet when you drop off the bar or rings. Smashing into the
ground face-first isn't fun for anybody except the people watching. So don't
do it. Land safe.
Other that that, the most important tip I can give you for performing the
back lever, is to lock your back and arms together. Below, anatomy man will
show you the muscles that you should be fusing together.

By keeping things tight, you will create tension in your upper body and
recruit the large muscles in your back, instead of just your arm muscles.
I've tried to demonstrate the difference in muscle tension in the following
pictures.

In this picture above, I am using my arm muscles when holding the bar, but
I'm am not using my back muscles as greatly as I can.

Here, I have flexed my arms and back muscles to create tension in my entire
upper body. As I said, it should feel like your arms are locked to your lats
(your latissimus dorsi). So in the end, this is
what your back lever should look like if shot from above. Use as many
muscles as you can. Why make it difficult by only using your arms?
The back lever really isn't that hard to get down. If you already have a bit
of pull up and dip strength, you might even be able to pull off this skill at
first go.
Good luck!