First and foremost, prior to cutting one must determine the correct angle to cut the mango at. Line up the fruit so that the narrow profile is visible to you so that the flat and wide side is held in your hand.
Place and Cut
Using the stem as your centre point, place your knife as close to the right side of the centre as possible to avoid the pit or seed of the fruit. Once you have determined a spot you are comfortable with, slice down with your knife.
If you hit the seed, remove your knife slice down slightly further out.
Repeat Other Side
Once you have made the first cut, it will give a strong impression of where and how large the seed is. Turn the fruit over, this time holding the centre and cutting the other side off.
Set the knife at the point you determine to be just outside the seed and slice down fully once again.
Score and Remove
Using your knife, cut into the flesh of each half of the fruit, making sure to cut the flesh but not pierce the skin of the fruit. You will need one cut from top to bottom and two across creating 6 similar sized pieces.
Use a spoon to remove these pieces from the fruit.
Repeat for the other half of the fruit.
If you hate wasting awesome fruit and wish to salvage what is left do as follows. Carefully remove the skin surrounding the flesh and use your knife to slice off the flesh around the seed.
It may be a little messy, but very worth it.