Be careful, work slowly and cautiously so as not to slip and score, chip or scratch the tile surface. Contingent upon the width of the actual grout joint, a variety of tools may work. Besides the grout saw tool, (the most commercially user friendly tool available), I've used an old style beer can opener with the pointed end (before pop tops, pull tabs were invented), an old screwdriver, a utility knife (up to three blades can be set at once to increase thickness of cut on some of the pro models), a dremel tool, or roto-zip. I found the best all round, all purpose tool for this is the cement backer board scoring/cutting tool made by QEP. The carbide tip and 3/32" (approx) thickness of the carbide works great on all but the thinnest of grout joints.