One of the most important steps in grouting tile is removing the wet grout from the surface of the tile. If you don't, the grout will dry and become rock solid on the tile. This will either leave globs of dried concrete grout on the tile or it will cover the tile in a thin layer of grout. Use a manual scraping process and remove the grout with a solvent.
Use a sponge drenched with water to soak the grout. Hold the sponge or lay it over the grout until the water softens it enough that you can scrape most of it away.
Scrape off the grout with a plastic scraper and a nylon scrubbing pad. You may not be able to remove all of the grout this way.
Mist the remaining grout with white vinegar. Keep the grout wet with white vinegar for at least 20 minutes. If you are removing grout from a wall this may require respraying the grout many times during those 20 minutes to ensure the grout doesn't dry out.
Scrape the rest of the grout with the plastic scraper.
Wipe the area with a water-dampened cloth.
Let the tile dry completely.
Never use metal scrapers as they will scratch the tile.