I used window film as a temporary window covering in my new bathroom while I figure out how to do something more permanent. Maybe I am not a precise enough person, but I cannot imagine cutting 4 lines straight enough to be satisfactory. I think it would be easier, like you say, with it on the mirror, but would be afraid a razor edge would scratch the mirror. Instead, have you looked at these borders? Also check out the section on corners. Depending on the look you want, maybe this would work

Here is a link that might be useful: Window film borders and corners

I used a adhesive-free decorative film border and matching corners on my bathroom mirror that was de-silvering. It worked wonderful framing the whole mirror. They have a large selection of beautiful designs and sizes. The best news is they will trim the film size you buy to the size you need, so you don't have to worry about cutting straight.

Here is a link that might be useful: decorative film

Yes, it can be used on mirrors and looks gorgeous. I've used it as a border, but cutting out shapes is another nice option. I would pre-cut the shapes before applying them. The design blog, TISLstyle.com is hosting a giveaway on window film - a lot a different patterns.

Here is a link that might be useful: TISLstyle.com