I'd say a damp cloth or something similar
You could also get a glass scraper and scrape it off, put something like a piece of cardboard underneath the place you're scraping so it doesn't fall into the tank
I have never used it myself, but I'd imagine that you could, considering it is film.
You can use white vinegar.
Yeah, you can easily cut it to size. I usually cut it with a straight edge and razor blade before application because it goes on smoother without having to fight the excess material. Window frosting is my go to and would make the style of your tank look great and add a layer of depth most backgrounds only limit. Just make sure you invest 10 minutes to watch a tutorial video on how to apply it best and without bubbles if you don't have experience with it. A hundred tiny bubbles between the film and glass looks pretty bad!
Good choice with the sand! I personally don't think enough tanks are done with just a natural tan color.
Frosted glass bgSorry black sand lovers, but in the end I went natural (forgive my betrayal), just because I'm going to try and do a Orinoco delta type scape with loads of wood / twigs and leaf litter and black water. I bought a sand which wasn't too bright, but I haven't seen it wet yet.
What background would look best with this?
Also thank you all for the opinions and advice, I really appreciate it