As soon as you sit down, unfold the napkin and place it onto your lap. At some fancy restaurants, the server will do it for you. The napkin should stay there until the end of the meal. If you need to get up at anytime, just simply place the napkin tidily off to the side of your plate.
DO NOT roll it up into a ball!
There is also no need to refold it into its original state. Just remember to place it back on to your lap when you get back! When you are done with your meal and everyone is ready to leave, simply place the napkin neatly to the left of your plate. Again, the napkin should not be folded completely or crumpled.
AT A DINNER PARTY:
Wait till the host, or hostess (the person throwing the party) unfolds their napkin first. That will be your cue to unfold your napkin and place it on to your lap. Again, the same rules apply as above.
Wiping your nose, blowing your nose, or wiping the silverware clean with your napkin.