First, find out which side of the mango is the meatiest by standing a mango on its end and letting it go. The side it falls on will be its meaty side.
Peel a mango with a vegetable peeler or paring knife. Then, cut a lengthwise slice down each side of the long flat seed as close as possible. Cut off as much of the flesh of the remaining two sides and discard the seed.
Cut the remaining mango pieces lengthwise into slices.
For mango cubes, start with an unpeeled mango, and cut the lengthwise sides from the seed.
On the flesh side, gently score each piece horizontally and then vertically, without cutting through to the skin.