Countertops and vanity units have overhangs that require extra attention in order to finish them properly -- that is, smoothly and with professional good looks. You'll need bull nose tile or v-cap edging for the job. Bull nose tile is flat, has one rounded edge and you'll need to pair it with trim tile. A v-cap is a v-shaped trim tile, usually with a rounded rather than a pointed corner. One v-cap tile does the work of the two-piece bull nose and trim.
Lay the field tiles first, then measure and cut the trim tiles to size. If you're using bull nose tiles, it's best to install a temporary batten on the edge of the counter's plywood base to ensure a straight line. Cut a piece of wood that is the same thickness as the trim tiles you'll use to cover the overhang. Then nail the batten flush with the top of the counter.
Apply mortar to the back of the bull nose tiles using a trowel. Set them in place on top of the counter base, front edge flush with the front of the batten. For the overhang, cut trim tiles to size, as necessary, and back butter with mortar. To keep them from sliding out of position, use masking tape to secure the tiles to the front of the overhang until the mortar cures (usually overnight).
Use v-cap trim as an alternative to bull nose tiles. Cover the counter base with mortar using a trowel. Back butter the inside bottom edge of the v-cap and press the v-cap into place. Sometimes the tile will extend below the counter overhang. If so, take care when grouting -- place your finger behind any open gaps to guide the grout into those backless channels.
Proceed with the tiling process as usual. Allow the mortar to dry before cleaning any residue from tile surfaces. Then grout the joints, allow the grout to set, and clean the tiles.
V-cap sometimes is slightly smaller than field tile. If this is the case, centre the cap between the field tile's grout joints. Doing so will result in slightly larger grout joints surrounding the c-cap, but it shouldn't be noticeable.