After finding the right spot on your window, take your etching and peel apart the two layers. You'll notice that one side is sticky and other is not.
Take the sticky layer and place it face down onto your window. Try not to cause any bubbling when doing this, it'll wrinkle the sheet.
Take the plastic applicator and scrape it across the surface of the sheet. You can use your hands too, but the applicator is a lot straighter and can push most of the air pockets out.
After that is done, peel away the top layer of the sheet. Make sure you do not peel away the etching. That should not happen if you pressed down hard enough using the plastic applicator.