The procedure is called rigging. Rigging a butterfly should only take about 5 minutes.

Take a nice image of a butterfly from the top and position it so the body is vertical with the head at the top.

1. Use the pen tool to create two masks. One for the body and one for the left wing.
2. Move the anchor point of the layer to the right edge of the left wing.
3. Make the layer 3D
4. Name the layer something like butterflyL
5. Duplicate the layer and rename the duplicate body
6. If you want to have antennae on the butterfly use the pen tool to create a couple of stroked paths. Make the shape layer 3D and make it a child of the Body layer.
7. Delete the mask for the left wing on the butterfly layer
8. Duplicate the butterfly layer and rename it something like butterflyR
9. Set the scale of the butterflyR layer to 0
10. Add the following expression to the Y rotation property of the butterflyL layer:     
    • value - thisComp.layer("butterflyR").transform.yRotation
11. Add the following expression to the scale property of the butterfly L layer:    
    • value + thisComp.layer("butterflyR").transform.scale
12. Set the rotation of the butterflyL layer to 180º
13. Move the butterfly R wing so that the left edge is adjacient to the right edge of the body layer
14. Here's the trick. Parent the left wing and the right wing to the body.
15. Select the body layer and press Ctrl/Cmnd + Alt/Option + o (Layer > Transform > Auto Orient) and set the oriientation to Orient Along Path.
16. Lock the Left wing and the antennae layer
17. Animate the y rotation of the butterflyR (right wing) layer. I usually use just 4 or five random keyframes and add loopOut(cycle = "pingpong") to the property
18. Lock the right wing layer.
19. Animate the position of the body layer.
20. Adjust the orientation of the body layer to line the body up with the path. 270º, 0º, 0º should work nicely

As a final step add a 3D null to the composition and make it the parent of the body. This will help you line up your butterfly in 3D space.

There you go. No need to pre-compose. If you are adding this butterfly to some live footage you can use AE's Camera Tracker to tie the butterfly to a null and it will be amazing. If you wanted to stop the flapping of the wingsa a simple pre-compose of this group of layers and a time remap with a new key frame where you want the flapping to stop would do the trick.

Here's a Project File (cs6 and 5.5) file with an example that took me about the same amount of time to create as it did to write this post.

Finnish one mask and start another one. You can put as many masks as you'd like on a single layer.

I just put up a short video on how to rotoscope that shows masking clearly. I'm just uploading the version with audio so it may be a few minutes before this video is ready.

Did you download the project file? Do so and then take it apart. I actually left out a step that was unneeded. You don't have to set the scale of the right wing to 0 unless you add the fancy expression that I left out to the right wing scale property.

Here, I've made a screen cap movie of the process. It's currently WO audio but it should give you an idea of the steps. It's just under 2 minutes. Enjoy.

By the way... the project link has been fixed. That's what I get for typing with my thumbs. You can also find a copy of the project Here. The second URL is a little friendlier with zip files.