Put them in the oven for 15 minutes. Do not open the oven or fidget with them during this time. Just let 'em cook. The smell will probably be too much to bear, so perhaps this is a good time to go to another room and focus on something else. :D
Then, take them out and flip them. I use my bare hands most of the time, but tongs work well too. (The first photo is after the first 15 minutes)
Once they're flipped, put them back in the oven for 10 minutes. :)
Both sides should now be dark brown and crispy and delicious. And hopefully spicy, if you followed my directions. Spicy is the way to go with sweet potato fries!
Keep in mind that I like mine REALLY dark and crispy. Should you want a lighter brown color, just reduce the cooking time a couple minutes for both sides.
Additional cooking tips:
these will not turn out very crispy at all if you're cooking anything else in the oven, or if the fries are crowded too close together. They'll sweat instead of crisp!
keep an eye on them because ovens are tricksy little things. You don't want them to get black.
also, a lighter colored baking sheet is preferable - mine are very dark and I've started to have to cut my cooking time a lot to avoid getting them too dark.
Feel free to cut them smaller and cook them for less time - 10 minutes for the first side and 8 for the second works well for thinner fries.