The only requirement for the subsurface of tile is that the surface needs to be relatively moisture resistant and level. The ideal surface is concrete or concrete backer board. However, existing tile provides many of the same benefits as concrete, making tiling over existing tile a feasible option. The downside to the project is the elevation of the floor. Additional tile increases the height by the height of the tile and the thin-set tile adhesive.
Lay out the tile on the existing floor to fit the tile properly. Adjust the positioning to limit cuts and to determine where cuts are necessary. Make any cuts needed to start the project, using a tile wet saw. Make additional cuts where needed as you install the floor.
Spread thin-set approximately 1/4-inch thick over a 2- or 3-square-foot area of the existing tile surface with the flat edge of a notched trowel, to level out the surface. Then drag the notched edge through the thin-set at an angle to the surface and also at an angle to the existing grout lines.
Press the tile into place starting in a corner. Position tiles in the desired pattern, placing tile spacers between each adjoining edge, until the section is complete.
Continue applying tile using the same steps, section by section, to complete the floor. Allow the thin-set to dry according to times listed on the container. Remove the tile spacers.
Spread premixed grout over the tiles with the flat edge of the trowel, pressing the grout into the gaps between tiles until they are filled. Clean the surface of the tiles and remove excess grout with a sponge dipped in a bucket of water. Allow the grout to dry for 48 to 72 hours before applying any form of sealant.
You will have to adjust thresholds to the tiled area to help guard against stumbles and falls caused by the higher floor level.