Once your application of floor tile grouting is completely finished, let the grout dry for about 20 to 30 minutes from the time you first began to apply the fresh grout.
Floor tile grout dries very fast, so you should work in sections to match your spreading speed, but ideally you’ll want to completely fill the floor tile grout lines in your full room before beginning to form the grout. If you are floor tiling a large room however, and find it to be impossible to complete the job in one go, then do the floor in designated sections of grouting and forming to prevent any problems.
In order to find out whether your grout lines are ready to be formed, take your finger and gently wipe it along the lines. The grout should be beginning to become hard with a top crust solid enough to take a good scrubbing with your sponge, although it should also still be soft enough that it does move and spread a little. If you attempt forming too soon, then you could remove more grout than you bargained for if it is still soft and moist. However, it you leave it too long to dry, then you will just give yourself more work in scrubbing, and will probably need a sponge with a harder surface to contend with the surface.
Before you begin forming, you should notice that your grout lines will have a raised form almost level with the top tile surface. Particularly when working with ceramic floor tiles which have edges that slope downwards, these grout lines need to be brought down to a slight dip to match the tile edge level closely.
With a fresh bucket of water and a good sturdy grouting sponge specifically designed for floor tiling applications, and not just any domestic use kitchen or bath sponge, soak the sponge in the water and wring out well. Taking your wet sponge, wipe over the grout lines in circular motions to remove the excess grout and form the shape of the lines. This method will give your lines a clean and uniform look, rather than grout spilling out over the edges of the floor tiles. Sometimes you may find that a straight wiping path initially may help at times, but the key is to maintain regular circular motions so as not to wipe out too much grout for a better finish.
Don’t worry about dry streaks appearing on the tiled surface at this stage, as you will be able to come back to them once the forming is complete. And always remember to regularly wring out your sponge in water, as you will find that it quickly gums up with tile grout and becomes unworkable after a few wipes. The water will become murky very fast, but at this stage it is not entirely important to keep changing for a clean solution.
One final pointer is that during forming you may find that dips have appeared in the grout lines, due to air pockets that managed to escape the correct grout spreading process. If these do occur, simply take some grout that you may find lying around on your floor tiles, insert it into the indent pushing it down and stuffing it in with your finger, and then form the line again by wiping over with the sponge. These do happen from time to time, so to be on the safe side, have a little extra grout handy when forming in order to fill them up where necessary.