First, you have to find the two ingredients:
White sweet potatoes are becoming more common; my introduction was at a Southeast Asian grocery. I have since found them at the chain natural foods store, and even the chain grocery down the street! The outside looks pretty much the same, but the inside is white, and it's not as sweet as the sweet potatoes you are accustomed to.
Now, the oil: you need unprocessed coconut oil because if it is processed, it is tasteless, and the coconut flavor complements the sweet potato. If the oil says that it is good for high temperature cooking, it is processed. If it says that it is virgin cold-pressed, it should be unprocessed. There are many more terms used; ask the grocery staff to help if you can't figure it out. You can find it on Amazon if you get desperate.
Now that you have the ingredients, wash and roast your sweet potato in the oven or microwave. (White sweet potatoes take to the microwave better than the orange kind.) While it's still hot, peel, cut into cubes (or mash it) and melt a tablespoon or so of the coconut oil over it. Enjoy!
Healthy: Coconut oil is healthier than butter on your sweet potato, and sweet potatoes are healthier than our more standard Irish potatoes.