Then use painters tape to connect the marks. Try hard to not bend the tape. If you pull the tape out gently and straight, you can trust that your lines will also be straight.
You can see here that the stripes don't look evenly spaced (because they're not). You'll need to tape above the registration mark and then below the registration mark to block out the full 8" for the stripe.
Then go back over the tape with your thumb and really smooth out and lay down the tape, just on the edges you'll be painting.
Here's the real trick for perfectly crisp lines: seal the edges of the painters tape with some of the base wall color before painting on the contrasting color. Don't paint the whole width of the tape. Just a bit along the edge of the tape on the side where the stripe will be painted.
Let that dry before you paint on the stripe color.
For my stripe color, I wanted to do a gray that sort of coordinated with the gray I just painted in my powder bath, since the doors are across the hall, only about three feet apart from each other. I didn't want to do the exact same color and I wanted something much lighter, so I mixed about one part BM Chelsea Gray with two parts White Dove.
You can roll or brush on the stripe color, and then I like to pull the tape off while the paint is still wet. I don't think it matters either way though, if you prefer waiting til it's dry. (#impatient)
Check out those lines! Not a bit of paint bled through because that extra little strep of sealing the edges with the base coat paint. I've used all brands of painters tape before too, and none of them give me the same results without doing the sealing step.
It's one of those completely satisfying projects that moves pretty fast. I busted out this closet in about two hours.
PS Some of you asked about the citron boxes I have on the shelf here. They're shoe boxes from Crewcuts! Ha! J. Crew is having an additional 40% off sale items plus free shipping, so how could I resist picking up some new spring kicks for the girls? The cute, sturdy boxes were a little bonus though, for sure. :)