Use a soft to medium brush on a slow speed rotary. Pre wet the floor and apply PSRP X-Treme Heavy Duty Tile & Grout Cleaner, agitate it with the brush and then allow it to dwell 10-15 Minutes, agitate it again then flood the floor , extract with a wet Vac , rinse and extract again. Allow it to thoroughly dry because that grout will take a while to dry ( use a snail dryer to speed it up). Then seal the floor with Miracle 511 H2O Impregnator. Then apply Miracle High Gloss Topical Sealer