Once the bond coat or setting bed has been trowelled, it is time to set the tile. Remember that it is a good idea to back butter tile larger than 8″ or any tile where 100% coverage is desired. Set the tile in place and beat it in using a rubber mallet and beating block. For small tiles up to 6″ use the beating block and mallet or just the rubber mallet for large tiles. The idea here is not to hammer the tile into place. The idea is just to firmly seat the tile into the setting bed. There is no need to exert great pressure on the tile. So "softly" beat the tiles into the bed of mortar.
Once the tiles are placed and beaten in, align the tiles with your layout lines or ledger and clean off any bond coat residue from the tile surface. The bond coat should not interfere with the grout ability to penetrate the joint by at least 2/3 of the joint depth.
At this point, the installer can apply spacers if necessary. When using spacers, do not put them into the corner joints where the four corners meet. Simply place them in the joint between the tile. They therefore stick out from the joint. Once the tile has set and the thin set has hardened the tile into place, simply brush off the spacers and re-use them. Removing spacers used in the traditional manner is difficult and the installer runs the risk of damaging the tile work in the process.
Once the tile above the ledgers has been installed, the ledgers are removed. The tile can be set below in the same manner as the tile above. If the tiles start to sag, they can be held in place with tape, masking or duct, until the bond coat holds them securely. The face of the tile will need to be dry in order for the tape to stick.