Firstly, press the "E" key to have your character flap it's wings and be off the ground (you only need to press this key once), secondly, press "R" to rise and "F" to fall (like you're swimming, except with wings in the sky, funnily enough), and then, if you want to plummet down to earth, you just press the "E" key again, this retracts the wings.
Hope that's enough detail for ye'.