A mango has a long flat seed inside which is the same color as the flesh. Use a paring knife to cut the flesh and skin from each of the flat sides as close to the seed as possible. Cut the skin away from the edge of the seed, and then cut off the flesh. Discard the seed. Score the flesh to the skin in the remaining sides. Push inward on the skin (turn inside out), and scrape off the fruit. It's now ready for chopping.
Make sure the cedar plank you use is untreated because of its direct contact with the food and so there are no unpleasant odors. Check local lumber suppliers; there's a good chance of finding this size piece in the "odds and ends" bin.