You also might have been worrying over it as you think this might cost you a lot to repair.
Read the simple step-by-step instructions below and watch this video tutorial byHow To with Kevinon YouTube as well.
The level and depth of your tool depend on the existing grout making sure you do not overfill.

Image credit: How To with Kevin via YouTube
*Thanks toHow To with Kevinvia YouTube for all images used here.
How To Repair Grout For Beginners

