The process of applying adhesive to the floor and the tile is refered to as "combined laying". Although I've never really come across a need to do this. You'll be fine just applying the adhesive to the floor - that's the accepted way of doing it.
What sort of tiles are you going to use?
Have you read the sticky at the top of the tiling forum?
You will need to butter the backs of the tiles as well as the floor if the tiles have button backs (large recessed squares / circles etc left by the die that presses the tiles at the factory). The trowel spreading thickness does depend on how flat your floor is and what tiles you are laying. For most man made tiles I would go with a 6-8mm trowel but you may need to spread the backs of the tiles to achieve good coverage.