On new tile walls, the correct way to install a ceramic corner shelf is to set it directly into the thin-set mortar during tile installation. But you can add a corner shelf (or another soap dish) on existing tile.
Stop by a tile shop and pick up a “flat-back” corner shelf unit (about $20). Then buy soap scum remover, double-face foam tape and a tube of silicone caulk. Clean off all the soap scum or the bond might fail.
Apply a strip of foam tape on each mounting flange, stopping 1 in. short of the ends. Test-fit the shelf before you remove the wax liner paper from the tape. Shim any gaps with additional layers of foam tape. Once the shelf fits squarely into the corner, apply the silicone caulk (Photo 1).
Locate a spot in the middle of a row of tiles (no horizontal grout lines running through the caulk). Then remove the liner, square up the shelf and press it against the tile (Photo 2). Once the tape grabs, let go, wipe off the excess caulk and then tool the joint with a damp fingertip. If you blow the placement, you’ll have to muscle off the shelf, completely clean off the back and reapply fresh tape, so be careful.