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.