Just wrapping in plastic wrap before microwaving will cook it unevenly, so in order to properly steam it, I tried wrapping it in a moistened kitchen towel as well.

If you microwave it for too long, you can't go back and fix it, so heat just a bit at first and then heat a little more at a time if it's still too hard.
The amount of time needed will depend on the size of your potatoes. Recipe by cook_hana