This seems like the simplest thing on earth to do, but it occurred to me that not everyone might think so, so here’s a mini lesson: grapefruit segmenting 101.
The single most important thing is to have a really sharp knife. I use a small paring knife usually. Start by cutting off both ends.
Then systematically going around the fruit with your knife cutting from top to bottom, slice off the skin and pith. You need to play a bit to find the right depth to achieve this without wasting too much of the good stuff.
Once the grapefruit is in its undressed condition, remove any big bits of pith or skin remaining. Then holding the grapefruit in one hand, cut along the sections, prizing out the pegs one at a time.