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)

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.

Step 3: Figure Out How Many Stenciled Images Can Fit on Each Wall

Your stencil should have “registration marks” in or near each corner. These little holes are used to ensure even spacing of your images. The image may extend beyond the registration marks, this is done to eliminate blank space on the finished project, and has no importance whatsoever for this step. Measure the distance between the registration marks and write that on your sketch.
Divide the length of each wall by the length of the stencil image. Ignore any remainders. Write the number of images that will fit on each wall on your sketch.
If you can fit an even number of images on every wall, go buy a lottery ticket because this is your lucky day.
Also, skip the next step.

Step 4: Find the Center of the Stencil

This step is only needed if you are going to put an odd number of images on the wall.
If you have an odd number of images, you need a center registration mark.
If your stencil has center registration mark already you can skip right ahead to the next step. Mine didn't so this project has more math.
If you are still reading, divide the distance between your two corner registration marks in half and write that down on your sketch.
As precisely as possible, use your tape measure or another ruler to find the center point between the two top registration marks.
Punch a hole at the center point with the hole punch. I used a small card making hole punch that I “borrowed” from my wife.
I also used a black marker to draw a center line down the entire stencil. This really wasn't necessary.

Step 5: Prepare the Wall

My ceilings are fairly level, so I just placed the top edge of the stencil along the corner where the wall and ceiling meet. If your ceilings are way off, or if you are stenciling at some height lower than a border, you could use either a laser level or a 48" long level to mark a level line around the room.

If you are adding a stencil to an existing coat of paint, make sure you remove all the loose paint, dust and cobwebs. Since I had just painted the room, I didn't have to do this either.

Step 6: Prepare the Stencil

Optional, but recommended. step - If there is a lot of extra plastic beyond the image feel free to cut it off. Make sure you leave enough plastic around the image to protect the wall from your brush, but too much plastic may smear your existing images. I trimmed a bit off of both sides of the stencil, which is why the sides look curved.

Not Optional from here - Spray the back side of the stencil with stencil adhesive, following the manufactures instructions about ventilation, dry time and temperature.
Pour a small amount of paint onto a paper plate
Tear off some paper towels and fold them into pads.

Step 7: Put the First Image on the Wall

Stick the stencil to the wall, aligning it with the center marks you put on the wall already.
If your wall will have an even number of images, line up either the left or right registration marks with the center mark on the wall. If you are doing an odd number of images, line up the center of the image with the center mark on the wall.
In either case, use your pencil to mark all four corner registration marks on the wall.
Holding the stencil brush straight up and down, tap it into the paint on the plate. Tap the brush onto the paper towel to take most of the paint off of the brush.
Bounce the brush onto the stencil repeatedly until the color is as heavy and opaque as you want it. I was painting a nursery, so I went fairly heavy. If I was painting a formal room, I’d probably go for something a bit wispier.

Step 8: Completing the Wall

After you've filled in the stencil, give yourself a few seconds to bask. Also give the paint a few seconds to set up.
Carefully peel the stencil away from the wall without smearing the wet paint.
Line the stencil up for the next image by placing the left pair of registration holes in the stencil over the right pair of existing pencil marks.
Use your pencil to mark the two holes for the next image.
Dab paint onto your brush and dry the brush on the towel.
Pounce the brush over the image until it is about as opaque as the previous one.
Repeat until you get to the end of the wall.

You are going to start at the center of the wall and move to the end of that wall in one direction. Then go back to the center and work toward the far side of that wall. I was able to place my step ladder so I could paint two images without moving the ladder, but it's still a lot of up & down the ladder and moving the ladder.