Here's a look at my process putting together a mosaic mirror.
I start by using mastic to glue the edge tiles on a wooden frame. That creates
a channel to drop my tiles into. The tiles are loose at this point. The frame
has a recess on the back side that the mirror will fit into.
Here I have all the tiles laid out the way I want them.
Next I like to use this broken Italian floor tile that has been tumbled to
fill in around the color tiles.
Finally the fun part - grouting. I find the black grout gives me the best
contrast and makes the colors really pop.