Is your plan to add the tile (i) to the wall around the mirror, (ii) to a wood frame that will be added around the mirror, or (iii) directly to the edge of the mirror in order to have the tile look like a frame?

You need to be mindful about how you attach the tiles because you will see a reflection of the back of the tiles in the mirror after they are applied.

I did see one person who used mortar on the mirror to attach the tiles - that seemed like a big mess + you'd see the mortar in the reflection of the mirror.

I actually used some mosaic tiles to create a frame around a plain square mirror many years ago. I can't remember exactly what I used to attach them to the mirror - all I know is that it was some type of adhesive that dried completely clear (+ didn't change the color of the back of the tile) so that you could not see any of it in the reflection of the mirror.

This one was made with a simple wood frame that the tiles were glued to instead of gluing them to the mirror. She also used pieces of wood that she cut and painted because she couldn't find any mosaic tiles in the colors she wanted (yellows).

She's actually pretty creative - and always provides detailed instructions. Here is the link:

https://www.addicted2decorating.com/bathroom-makeover-day-14-diy-mosaic-wood-tile-mirror-frame-finished.html

Thank you! Will be wall mounting next to the mirror rather than on the mirror itself. Mirror is an LED
dimmable anti fogging so I hate to put something on it

Might need to have the mirror recessed into the wall.