The big thing to remember is always twist your spindle the same direction, to the right. If you start twisting it the other way, you'll untwist your spun yarn, the wool will pull apart, and your spindle will fall to the floor. Your spindle's going to hit the floor pretty often anyway for various reasons (another reason to get a sturdy three-ounce spindle), but as long as you twist to the right you'll be making yarn.
Stand up. Hold your spindle by the shaft in one hand (which hand doesn't matter; I'm right handed and use my right hand, but you may find it more comfortable the other way around), and hold the leader in your other hand. Pinch your fingers down over the slipknot. The rest of the roving should be out of the way; many spinners wind it loosely around their wrist, some drape it over their arm and let the end hang. You'll find your own way once you've had some practice.
Give the spindle a sharp twist--to the right--and let it go. It may bounce around a little on the end of the leader, but it should keep turning. If it doesn't, try it again. You'll be able to feel the leader twisting up; that's the twist that's going to turn the roving into yarn.
Replace the hand holding the slipknot with your other hand, and move your other hand back along the roving. Now open your fingers on the slipknot just enough so that some of that twist runs up into the roving. Congratulations! You've just spun your first few inches of yarn!
Yes, it's that easy. Keep moving your hands backwards along the roving, letting more twist into the wool. Your hand in front adjusts how much twist you let into the roving so should be tighter on the wool than your hand in back, which is around the wool loosely. Use both hands to tug any thicker spots in the roving apart just enough to make them consistent with the rest of the roving. This is called drafting, and it'll become second nature before you know it.
Keep an eye on the spindle. When it slows down, pinch the fingers of your back hand together to stop the drafting process, and reach down with your front hand to give the spindle another sharp twist (to the right). Just keep going until you've spun so much yarn the spindle is down to the floor. Then pick the spindle up, unhook the leader, and twirl the leader and your newly spun yarn onto the spindle shaft, leaving enough yarn free to run up under the hook (or half-hitch to the notch), and then do the whole thing over again.
That's spinning. It may seem awkward now, but practice is the key. It's just like learning to drive a car--a lot to keep in mind at first, but with a little practice it all becomes automatic.