Simple enough if you can be patient, steady handed and have the right tools. Make sure the glass is spectacularly clean. Not given a wipe clean, given a proper alcohol based cleaner clean and run a decent blade over the surface to get any tiny grits off. The tiniest little piece of grit will look like a bubble when the film is applied. Dont try and cut it right up to the window edge, cut it 5mm short for a better, sharper finish. If you've got spare, practise somewhere less visible first, just to get used to what the materials do. Gets a life of it's own if it's charged with static. Have a sharp knife, a sharp knife blade, a squeejie, squirty bottle of water / cleaning fluid, a few rags or sponges before you start. Change your blade before it's blunt not once it's proved it's gone blunt. But yeah, not the hardest job in the world. Unless you've put patterns, lines, wording and shapes into the design!