How do you frame a 7ft mirror that doesn’t already have a frame?

If mirror is fixed to wall - cut wooden frame and stick to wall - Use wallpaper border. Tiles - or Mirror Mosaic. etc............

https://www.diynetwork.com/how-to/rooms-and-spaces/bathroom/how-to-build-a-frame-around-a-bathroom-mirror-pictures

I have bought molding from the hardware store and mitered the corners. If you want a temporary fix or the mirror is fixed to the wall, you can use a temp. or permanent adhesive right on the mirror itself. If you can remove it, then I would cut a thin plywood a little larger than the dimensions of the mirror, and mount the mirror to that before adhering the mitered pieces to the plywood around the edges of the mirror. You will have to determine how much wider the wood needs to be to accommodate the molding. You can use putty to seal the cracks at the corners, and paint for a seamless look. (Your mounting hardware would need to be affixed to the wood, probably prior to affixing the mirror to the wood.)

Here is one way to do it - http://www.momtastic.com/diy/128900-how-to-frame-a-bathroom-mirror/#/slide/1

You can get barn wood and cut it to length and put it on the edges.

My mirror was already attached in the bathroom. I nailed the vinyl trim. Looks great.