Take blue painter's tape (or any tape that won't damage your wall) and stretch it across the back of your artwork. Mark the tape with a pen at each sawtooth hanger.
Put the tape on your wall and eyeball it to make sure it's straight. Place nails or hanging hooks above the marks on the tape. Remove tape. Hang artwork on nails or hanging hooks.
**Note, these directions are for my artwork, however, they also explain how to how to hang pictures straight without a level and how to hang wall art straight without a level that is not mine.