Dice up your potatoes so they are all about the same shape and size. You can leave the skins on. They should be about the size of a quarter (size doesn't matter as much as uniformity).
Heat your skillet (with no oil in it yet) on medium high heat for 2 minutes. Your skillet should be well seasoned (see how to do this here). Now add your oil (about 3-4 tbs) and let it heat for about 2 more minutes. Make sure you use a high smoke point oil like avocado or coconut oil.
To test the oil to see if its hot enough, toss in one diced sweet potato - if it sizzles then its ready - if not, wait another minute or so.
Make sure you don't overcrowd the cast iron skillet. All the potatoes should fit in a single layer on the pan. You should hear a beautiful sizzle!
Resist the urge to turn the potatoes for at least 3-5 minutes (timing depends on your pan, how large dices are, etc.). Leave them on this side longer than you think you should.
Only flip one sweet potato to see if it has formed a brown crust. If so, you're good to flip - if not, wait a couple more minutes.
When this side has formed a brown crust - go ahead and flip the potatoes by using a spatula to toss them all around. Put them back in a single layer and make sure the brown side is up now.
If your potatoes are releasing a lot of moisture, you might need to increase the heat slightly to help it evaporate more quickly (this ensures you don't steam the potatoes). If they are browning too quickly, turn the heat down. Play with the heat as you go!
Repeat the flipping process as needed to make sure all sides are brown and crispy. Try to flip as little as possible though until a nice crust has formed around the whole diced potato.
Save seasoning your potatoes until the very end. Salt releases moisture in your potatoes and moisture will steam your potatoes rather than make them crispy. Season with whatever sounds good to you. I like salt and cumin. Or garlic salt.
Scoop the potatoes out of the pan and serve them up. They will be very hot so make sure you let them cool before devouring!