Summarize the following:
Apply tile mastic or quick-set mortar to the area around the outlet, being careful not to get any into the electrical box. Push the cut tiles firmly into place. Use appropriately sized tile spacers to keep consistent space between the tiles as they set. You have to finish grouting the space between the tiles before you can tighten down the electrical outlet in the box. You may have to hold the outlet in place as you tighten to stop it from twisting and misaligning with the faceplate. Once the tiles are installed and grouted, replace the outlet cover. The outlet cover should hide all the cut edges of your tiles.
Install the tiles around the outlet. Apply grout between the joints. Tighten down the electrical outlet. Replace the electrical outlet cover.