I kept this pretty simple so you wouldn't be cussing me during the process :)
Use either blue painters tape or Frog tape for masking lines after using a level and pencil to mark the walls. Be sure to run thumb along edge of tape to get a secure seal. If you have a steady hand, skip the tape. There is always a bit of touching up with the tape which can be avoided if you don't use it.
Be sure to give your base coat a few days to cure before masking for the stripes.