Most of the rooms in our house have crown molding, which I don't care for in a little girl's room, so I decided to try my hand at a painted stencil border.
This How-to should either give you enough information to proceed confidently or decide that this isn't for you. (I'm kidding, if you can paint a wall, you can add a stencil border)
Step 1: Sketch the room
Step 2: Find and mark the center of the wall
Write the length beside each wall on your sketch.
Divide the length of the wall in half to find the center.
Write the “half-length” of each wall on your sketch.
Mark the center point of the wall, near the ceiling, with the pencil.
Repeat this process for each wall. Don’t assume that opposite walls are the same length, because they probably are not.