Instructables
I recently painted the back room of my house in anticipation of the birth of our first child. Veronica was gracious enough to wait for me to finish decorating her room before she joined us in the outside world.
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)

 
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Step 1: Sketch the room

Draw a rough sketch of the room on a piece of paper. You’ll use this to keep track of your measurements and calculations for each wall.

Step 2: Find and mark the center of the wall

Measure the length of the wall in inches and fractional inches. Ignore the number of feet.  (For example, a wall that is just shy of 10 feet long might be 117 ½” long. You don’t care that it’s 9 feet, 9 ½ inches)
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.