We always recommend cleaning (or stripping) your tile and grout prior to sealing. Click here for more information about grout cleaning.
Many people find grout sealing to be difficult, but it couldn't be any easier:
To seal the grout lines between ceramic, porcelain, and quarry tiles, we recommend sealing once a year with Seal & Go® S or Seal & Go® W, water/stain repellent penetrating sealers. These sealers will not alter the natural appearance of tile or grout when applied correctly.