Use a grout float to press the grout into the space between tiles.
How to grout slate floor tiles.
After both sides of your sponge have grout on them rinse your sponge off and repeat the process until all joints have an appropriate amount of grout.
Once you have done this only use the other side of the sponge and only once.
By sealing your slate tiles you are blocking the pores of the stone this will then enable the grout to float over the pin head sized holes on the stone not stick within the pores the grout should now be easily removed when it is applied you are now ready for grouting the slate tiles.
Scoop some grout onto the tile floor with a trowel.
To avoid spreading the excess grout across your tiles use one side of the sponge to wipe along the joint.
Then start to apply the grout into the lines and joints in between the tiles by pressing it downwards.
Select an area 2 to 3 feet square to begin and apply the grout with a rubber faced trowel to spread the grout across the slates working it into the channels between the tiles.
Scoop a small amount of grout on a grout squeegee.
Scrape away any excess grout.
Begin in the corner farthest from the doorway and work backwards.
Fill a plastic spray bottle or a small pump sprayer with grout and tile sealer and spray the slate tile evenly making sure to get the sealer on the inside edges of the tiles.
It is best to use unsanded grout for slate floors.
Press the tile into the opening on the floor.
First move the grout float a specially designed smoothing tool that features a flat and mostly firm rubber pad.