Why You Do ThisYou will force wet grout between the seams of the tiles. After a few hours, the grout hardens. Without grout, you will have structurally weak tiles and unsightly seams that will collect dirt.
Amount to UseDon't put on too much. My motto: the more grout you lay down, the more you'll eventually have to scrape off.
I like to slop about two index finger-sized portions of grout onto the edge of my float.
DirectionFirst, press it into the seams in a diagonal fashion.
Before that, though, you may need to move parallel to the seams, simply to get the grout deeper into the seams. But once the grout is in place, you must switch to diagonal sweeps.
Use Edge of FloatRarely will you use the "flat" of the rubber float. Most of the time, you will be using the edge, in the same way you might use a squeegee to clean a window.
Grout Across Entire Tile SurfaceDo not worry if you scrape grout across the surface of the tile.
On smaller tiles (six inches or smaller), it is impossible to maintain contact only with the seams. That's the purpose of the rubber float: to both press the grout into the seams, and to scrape excess grout off of the surface of the tiles.