Where are you?
If you have access to real tile stores you can go mooch some "Noble CIS Mat", or KERDI Mat. These products are intended for crack isolation when installing ceramic tile over concrete (among other great uses.)

They both have a "bonding fleece" attached to them for tile application. Neither are 1/8" thick but it's a beginning. Once you have the tiles applied to the mats you can then use thinset to comb lines on the wall where the tiles will go. Once the lines are dried you can then use sandpaper to reduce the elevation of the lines of thinset to the needed height.

Another way is to cut the mats (with tile attached and dry) into 12" lengths. Thinset can then be applied to the backside of the mats and laid flat upside down until the thinset dries. At that time the thinset can be sanded to the proper height.

This isn't as difficult and complicated as it may sound. I have done this a lot with great success.

Thanks!

A guy at a local tile store gave me a hunk of orange kerdi last night, and I thinsetted a couple of the tiles to it. They peeled right off this morning. The thinset stuck to the tiles, but not to the kerdi. Did I do something wrong? I worry that the Kerdi won't stick, with such a small surface area.

It'll stick. You need to give it more than just overnight!!

DON'T TRY TO PEEL IT OFF! Once it is "set" on the KERDI and the tiles are in place on the wall and grouted they won't be going anywhere. TRUST ME!

This is what I did in my kitchen and it came out perfectly (the tile guys had better close their eyes now).

I went to the craft store and bought some 1/8" thick balsa wood strips and I drew lines on my wall where I wanted the glass tile stripe to be. I then tacked up the balsa wood to mask the stripe. I used drywall compound to build up the 1/8" thick stripe because it was my backsplash and wasn't going to get wet but if I were doing this in a bathroom I'd use thinset. Basically I used a flat putty knife to fill the space between the two pieces of balsa and when I was done and it was dry I had the perfect "platform" to lay the glass tile on.

I did it this way because I knew I did not have the practice, skill, or the "touch" to build up thinset on the fly under the glass tile. I do have pictures of my method somewhere if you need them, I'd have to dig them up but I can do that. I was REALLY pleased with how it came out - the thinner glass tile when set is perfectly flush with the thicker field tile.

Creativity often saves the day and I can't see why what you did there would be critized at all for a DIY'er. Not only a good solution, but it sure looks beautiful. And you get to brag about how you figured out how to do it. Very good job.

Thanks!

Yes, one idea that I had (that's still a work in progress) was to create the eight inch deep, 1-3/8" wide strips of thinset on the kitchen table, and then thinset the mosaic tiles to the strips, and then carefully place those "mosaic tile strips" on the wall. I think that will work too, but it's all so delicate until its on the wall, because thinset isn't really strong when its that thin. (of course, that's where the Kerdi could come in!)

The issue is that the tiles are so tiny, and I don't want to end up placing the thousands of tiles on the wall one by one. I'd much rather sit in front of the TV and do that. I'm actually taking them off the mesh, so I can put them into a narrower strip.

Thanks for your help! I'll keep you posted, if you like. My big fear is that after I do all this work and its on the wall, I won't like it.

So, what kind of thinset should I use to adhere the tiny thin glass mosaic tiles to the Kerdi?

My husband has been using a modified thinset to put the ceramic tiles over redgard - can I just use that?

I don't see why not, even though Schluter says you should use unmodified. I use modified thisnet over Kerdi any time I install Kerdi walls, and have yet to have a problem from it.