Watch the tutorial here: https://www.youtube.com/watch?v=DCxSxbx6oG0
How to sequence layers to create backgrounds and transitions:
Create your composition, then add a new solid layer. Then, ADD (not mask) a shape layer and choose a fill.
For this example, we will create sort of a stairstep animation with tall, thin rectangles. The shape layer you just created, in this case, should be one of those rectangles.
Let's say you want to do 10 of these rectangles in succession, and they should all be the same width. Leave your calculator on the desk. Go to your width property (aka the left side of the size), and you can literally do the math right there. If your comp is 1920px wide and you need 10 rectangles to fit perfectly, simply type in 1920/10. BAM. Magic.
Adjust your anchor point accordingly. For the example, we'll anchor it in the bottom left corner.
While you still have this one rectangle, animate it. For example, let's say you wanted the scale to change on all the rectangles as they show up. Animate that scale change on your single rectangle.
Then, duplicate and reposition it as necessary. This way, you won't have to keep copying and pasting keyframes.
Since these rectangles are the same width, repositioning is easy. They are each 192px wide. The first rectangle is already in place. The second rectangle will be positioned at 192. The third rectangle can be repositioned using an expression (192*2). Same with all the rest.
In order to sequence layers, you must select them all. To animate from left to right (since we started on the left in this example), select all your layers from bottom to top. If you select from top to bottom, it will animate from left to right.
Once your layers are selected, go to animation > keyframe assistant > sequence layers.
If you want them all to end at the same time, move the last keyframes to the same spot.
Now, say it's going too fast for your liking. To extend the length of time for all of them at once, select all keyframes. Then hold the option key and drag. This spread them out while maintaining the relative distance between them.
He plays around with more effects in the video, such as pre-comping this, then duplicating it, flipping it, and changing the color. For more fun stuff like that, watch the video.