Picture Frame Miter
The standard picture-frame miter is when you have a corner such as around a window frame or the outside edge of a countertop where two pieces of trim need to wrap the corner. A standard 90-degree corner wrap is the easiest to make, since you merely need to cut the ends of the adjacent pieces off at 45 degrees and set the pieces together with a grout joint between the two pieces. For ceramic tile or porcelain, use a tile-cutting board to cut the ends off and then a rubbing stone to smooth the edges. Otherwise, use a tile wet saw and associated miter gauge to cut the ends off. Always use a block of wood to help hold V-cap bullnose -- trim with a molded drip edge -- in the upright position while cutting.
Inside/outside Corner Miters
When you are working with accent trim, such as quarter-round, cap molding, liner bars, liner ropes and V-cap, you sometimes have to wrap an inside corner. Traditionally this is a 90-degree corner, but the angle may be different depending on the layout of the home. For a traditional house, the two pieces meeting up in the inside corner need to be cut off at 45-degree angles to make the 90-degree turn at the inside corner. Use a tile wet saw to make the cuts. Adjust the angle of the miter accordingly depending on the angle of the corner and the adjacent walls. The same goes for outer corner wraps, except that you reverse the direction of the miter, since you aren't folding into a corner, but rather wrapping around an outside corner.
Toe-kick Base Miter
Rounded toe-kick base trim, which has a toe that kicks out at the bottom making the tile look like the shape of an "L," is mitered with tile nippers as opposed to a tile wet saw or tile cutting board. Choose the least-visible wall and “bury” the back wall trim pieces into the corners behind the piece of trim from the perpendicular wall. For example, the back wall of the shower has its cuts buried behind the two side walls so when you are standing looking into the shower you don't see the cuts running up the wall. Nip a slight corner out of the bottom edge of the toe-kick base tile for the adjacent, perpendicular wall so it installs over the top of the bottom section of the back wall’s toe where it juts out from the bottom of the "L." This hides the actual miter from view unless you are standing facing the side wall, where you look down and see the notched-out miter section covering the toe of the back wall’s trim.
Keep an angle finder handy if you are working on homes with non-standard angles to help you determine how to cut your miters. Always cover the area of the tile trim you want to cut with a layer or two of masking tape before running the tile through the wet saw to avoid small pieces chipping off. This is especially important for smaller tiles and rope trim, or for glass tiles, as the point of the miter will chip off without tape. On the off chance that a piece does chip off, keep a bottle of super-glue handy to glue the piece back together. Always use the gauge on the tile wet saw to help keep vibrations to a minimum and hold the tile in place during cutting for higher precision.