Hi There! I would suggest wedi board or some kind of cement backerboard, you can do your mosaic on it, then screw it to the wall. Just go to a flooring/tile store and ask about this, I find them, most helpful, as they are realizing there is more and more home owners that are wanting to do these projects themselves.

great Kitchen, love the wall colors. I agree with Jane, cut cemnt board to fit and find your studs for the screw holes. Then you can build it laying flat. Just remember if you have any switches or outlets in the space you need to extend them out

I agree with the others. If you wanted to you could frame it out with some trimming from the lumber yard too...just to give it a finished look.

Be sure to post photos when you are done or even keep us up to date with work in progress... those are fun...and your house looks brand new...lovely

Or you could use plexiglass like I did on my stair risers. I drilled a hole near each end, mosaiced and grouted before installation, and then just screwed each one into place. Here's another thought. For your edges you could use the edge pieces of ceramic tile, some are called bullnose tile I believe, so that you have a finished look to the edges. Keep us posted, please!

Here is a link that might be useful: Silva's archived blog posts on stair risers