Paint your base color. Make sure you leave plenty of time for it to cure. This is a process that should not be rushed.
Using your laser level and tape measure, start measuring, marking, re-measuring, adjusting, re-measuring and
USE FROG TAPE. Do not skimp and buy the 3M or generic brand. I have used both and it is an unbelievable difference when it comes to paint seeping under the tape. I will repeat, use Frog Tape!! Be sure to also put the tape on the inside or outside of your mark depending on which area needs to be painted.
Use tape or some way to indicate which stripes don't need to be painted so you don't accidentally start painting the wrong area.
Here's the biggest tip I can offer. On the day you're planning on painting, take the original base color and paint a strip (an inch or so tall) half across the tape and half into the area you're going to be painting the new color. This helps seal the tape down and IF there's any seepage, it's going to be the base color seeping onto the base color. No big deal. Read the paint can and let dry for the hour or whatever it says before you begin painting your new color.
You want to do this step right before you paint the new color so that it doesn't have time to cure and you risk peeling or chipping the paint. Minimal drying time is all you'll need before you can paint over it.
Go ahead and paint your new stripe color. You're going to want to get all the necessary coats in on one day. Let the first coat dry for however long the paint can indicates and then get your second coat on right away. This is for the same reason as above... you're not going to want to risk cracking or peeling if the paint sits too long on top of the tape. Remove the tape SLOWLY at an angle as soon as you're done with your last coat.
We were extremely surprised with how clean the lines turned out. We only did two walls in a pretty tiny room and it was fairly time consuming. Measuring and taping took roughly 2-3 hours and painting two coats (with dry time) took most of the day. It's so worth it though. I absolutely LOVE how the nursery turned out. Check out my nursery reveal post to see how it all came together!