Hi Woody
When tiling with mosaic, tiling should always start at eye level (obviously depending on what youre doing). This will be the most focal point and its important to keep these lines running straight.
Once youve got the height, mark a horizontal level line at the heighest point (your datum), and then draw your vertical line.
Use these datum lines as the point to start.
You dont mention the size of the tesserae(10, 12, 20 mm etc) but with the small ones the surface must be flat as the slightest lump will put the tesserae out of bonk. You dont say what your tiling but ideally it should be flat, even and square with no cuts (with the posh mosaics).
Use a 3mm notched trowel. Mapei is good for mosaics, I know a couple of mosaic companies that use it.
You can use an angle grinder to trim the tesserae. If theyre glass you can also use a glass nipper.
If there are cuts, as with odd shapes, they shouldnt be on the outer edges.
Are they on a mesh or paper?
With mesh the thicker line should run vertically as the thinner line is easier to cut with a small knife when its on the wall. The same with paper the lines should run vertically. This is so you can space out the columns to fit.
have fun!
missy tiler