If you want your leftover fries to be crispy like they were when you bought them, a microwave isn't the appliance to use. Microwaving fries makes them warm, but limp and soggy. If you don't mind this texture, simply place them on a microwave-safe plate, cover the plate loosely with a paper towel and microwave the fries on high-heat at 20-second intervals until warmed to your liking. Better ways to reheat fries exist, though.
Microwaving encourages moisture to be released from food, so it works great when reheating soup, braises or other juicy foods -- but terribly when you're trying to recreate a crispy crust and soft center. The nuanced texture of fried foods, such as french fries, is beyond a microwave oven's capabilities.
A toaster oven or regular oven provides a superior way to reheat french fries or any other fried food. To use a toaster oven:
Step 1: Spread Out the Fries
Place the fries on a small metal baking tray and spread them out in one layer. Avoid crowding them on the tray.
Step 2: Preheat the Oven
Set the temperature to 350 F. Any hotter and the fries may burn before they heat through.
Step 3: Reheat
Slide the tray into the toaster oven and reheat the fries for 5 to 7 minutes. Check them every few minutes to see if they've reached the desired temperature.
Step 4: Remove
Remove the fries from the toaster oven and eat immediately. If they cool off, they'll turn soggy again.
Use the same method for reheating fries in a standard oven. If you have a large batch to heat, you can use a larger tray and do more at once. Still space out the fries to maximize crispiness. If they're too close together, it encourages steaming.