How to incubate an egg?

a duck decided to lay her eggs in front of my house and something ate them all but one so i am currently incubating it by having a heater on it ( temp varies 98-100 F) and i wrap it in a damp paper towel for humidity is there any thing i should be doing differently? Also i was wondering if anyone could help me figure out if its fertle or not, when i candled it you could see a dark circle maybe the yolk and it also had a small floating circle inside the bigger one. the egg is only two days old it was laid yesterday