Mosaics are small tiles measuring 5 cm (2 inches) or less in size. They can be made of any material, such as glass, ceramic or porcelain, and typically come ready for installation, mounted on a sheet of fibreglass, paper or plastic with built-in grout lines. When tiling two walls that meet in a corner, these grout lines can help give you a clue as to how much room you should leave open at each corner for the mosaics to have an expansion joint. While this may require trimming the mosaics slightly, the end result will leave the cut tiles hidden from view.
Measure each wall where the mosaics will be tiled separately and find the centre point of the wall. Mark this line vertically from top to bottom with a straight edge and pencil or chalk. This is where you will lay your first sheet of mosaic, moving outward toward the edges of the walls so the cut tiles are balanced evenly on each side.
Lay out your mosaic tile on the floor in front of the walls to get a feel for how far apart the sheets need to be and how many sheets will fit on the installation. Leave a minimum of 3 mm (1/8 inch) between the last row of mosaic tile and the corner on each wall for a minimum 6 mm (1/4 inch) expansion joint in each corner.
Use a utility knife to cut the mosaic sheets as close to the correct size as possible for the corners. Use tile nippers to cut out individual mosaics, if necessary, to fit the corners of the installation.
Spread thinset over the wall, beginning at the centre point and smoothing it out with the flat edge of a trowel. Press each sheet of mosaic firmly into the mortar, smoothing it out by hand until even or place a 5 by 10 cm (2 by 4 inch) board across each sheet and tamp the mosaics in with a mallet.
Continue laying the mosaic sheets from the centre out to the edges or corners of the installation. Spread mortar onto the backs of the sheets placed in the corners, rather than on the wall, and smooth them into place. Wait 24 hours for the mortar to dry before grouting.
Smooth grout over the entire installation with a grout float, packing the grout between the many joints in the tiles. Do not attempt to grout the corner joint where the two walls meet; this will be caulked. Wipe away the excess grout with a damp sponge; the grout will dry within 24 hours.
Cut the tip off a tube of caulk with the utility knife to approximately the same width as the corner expansion joint. Insert the tip of the tube into a caulk gun and tighten down the plunger until caulk issues forth when the trigger is pulled. Run a thin bead of caulk down the corner joint between the two rows of mosaics. Moisten your finger with water and smooth out the caulk, pressing it gently into the mosaics on either side to finish.