How to prevent the floor tile from appearing figure-eight?

In the process of sticking floor tiles, it is necessary to stick them with lines to ensure the flatness of the floor. Prevent the floor tiles from opening. Before sticking tiles, soak them in water and apply tile adhesive on the back, so that the tiles can be better combined with the cement mortar below, otherwise it will be difficult to make large tiles completely flat and there will always be bumps. If there is no adhesive, there is a gap between the cement slurry and the tile, which may be empty in the future.