Be careful to avoid scratching the shower pan; you might want to cover the pan edge with masking tape first. When the grout is gone, clean all the dust and other debris and let the surfaces dry ...

then scrape any protruding grout using a stiff putty knife and vacuum again. Don't scrape too hard or you may chip the tile ...

Then use a putty knife to carefully scrape any lumps of old grout, and vacuum again. Finally, tape the border to avoid getting grout on a painted wall, trim, bathtub or other finished surfaces.

Because these are small cuts, it’s easy enough to do this with a manual grout saw. Tape the tile and fill the gaps you’ve cut with silicone caulk. Fit the shelf into place. The tabs will hold ...