Dull the surface of the floor tiles, one by one, with your belt sander and aluminum-oxide sanding belt. Run the sander over each tile in a consistent direction until it's no longer shiny.
Wipe up the dust with tack cloths.
Brush polyurethane over each tile in a thin, smooth coat, starting at the far end of the room so you can work your way out. Keep the polyurethane on the tiles themselves and not the grout lines.
Let the polyurethane set for 12 hours.
Sand the polyurethane lightly by hand using 240-grit standard sandpaper, just enough to dull the shine. Wipe up the dust with tack cloths.
Apply a second layer of polyurethane to the tiles in the same way as the first. Let it set for 12 hours. Dull the shine with sandpaper. Wipe up the dust.
Brush on a third layer of polyurethane. Let it dry 24 hours.