Whether you're planing to dice your watermelon for fruit salad or slice it for a mirror display, your first step is to remove the rind or "peel". Just like with any other larger fruit, start by topping and tailing and then stand on one end.
Following the contour of the watermelon, slice off the outer rind just down to the red flesh. Don't feel like you have to get it all in the first pass; you can always go and trim off more, but it's impossible to put chunks back.
Slice the watermelon into quarters lengthwise and lay on its side. Cut into even slices, about 1/4"1/2" thick, using your index finger to hold the melon slices together. When sliced properly, you'll be able to fan the watermelon and have it stand on its own.