Measure the dimensions of the electrical outlet with a measuring tape with the electrical cover off.
Draw these dimensions on a piece of paper. Draw not only the width and length of the outlet, but the dimensions of the entire space surrounding it.
Tape the paper to the tile, and use it as a stencil in which to cut the tile.
Drill a hole into each corner of the stencil through the tile using a drill with a diamond-coated bit.
Cut the rest of the shape out of the tile using a diamond-coated saw blade in a tile saw, circular saw or grinder.
Remove any remaining protrusions from the tile with tile nippers.