Place the grapefruit on the cutting board so that the top and bottom of it are on either side and the actual side of the grapefruit is on the board.
Slice the grapefruit in half so that you have a top and a bottom.
Place them onto the cutting board skin side down.
Use the paring knife to cut around the perimeter of the grapefruit so that the peel and white pith is separate from the fruit all the way around.
Now, starting at the middle of the fruit, cut on either side of each white line membrane so that you're effectively loosening each wedge of fruit from the membrane.
Sprinkle with a bit of sugar and enjoy! The fruit should come right out with very little effort.
Notes
This can be done the night before and stored in the refrigerator. Be aware that citrus tends to be a bit more tart when refrigerated.