Use small shims or other spacers to allow a small amount of space between the bottom of the tile and the lip of the shower pan. Small pieces of cardboard work great as makeshift shims. After the mortar is set, you can simply remove these shims and caulk between the lip of the pan and the bottom of the tile.
Continue laying the tile, making sure to use spacers in between tiles. Spacers will leave you identifiable grout lines between tiles. Small 1/16″ or 1/8″ spacers (or even bigger) can be placed at several places on the x- and y-axis of the tile to leave adequate space for an even grout line.
Repeat the procedure, setting each row of tile on top of the last row of tile. Continue until you reach the top of the shower stall. Measure up from top of the last row. Mark the line with a level as a guide for your next row. Spread thin-set onto your tiles with v-notched trowel and place the tile just below the marked line. Use spacers all around.
Make sure to measure every third or fourth row to ensure that you have an even grout line. Let the tile set for 48 hours. This will make sure that the mortar has adhered properly to both the tile backing and the cement board backer. Do not use tile nippers made from slate. You can use them on ceramic tiles, however. You can also use a wet saw to make basic cuts on most tiles.
Cut the tiles while they are still on the mesh. Using your sketched line as a guide, cut the tiles down to the right size. A wet saw should do the trick for most tiles. If the tiles are smaller than 1 or 2 inches (2.5 or 5.1 cm), a pair of tile nippers may work better. The tiles should fall away from their mesh backing as you cut them. If they don’t. cut them away with your box cutter.