Stick your design to the paper side of the frosted plastic with pritt stick
Nows the tricky part, you'll need a steady hand and a sharp scalpel
Stick your design onto another printout of your design with the plastic side down using pritt stick again. This will make sure your design is all lined up properly and it will help when it comes to applying it.
Peel off all the backing paper from your design, now you're ready to apply the etch to your window.
Tape a mirror image of your design on the outside of your window, this will help you line the etch up.