The process of applying a decal to glass is relatively simple. It can be done with the following steps:
Clean the surface of the window or mirror thoroughly. This will remove any dirt, grease, oil, and other residues interfering with adhesion.
Apply an even coat of liquid dish soap to the surface you will apply your decal film on; this will help reduce air bubbles in between layers when you peel off the backing paper later on.
Peel the backing paper off of the decal.
Once the backing is removed, slowly and carefully place the decal on your desired surface.