By doing so yourself. Eat in a tidy fashion and be the narrator of your actions and thoughts. When you are finished eating explain how you are cleaning up and why you are cleaning up. If they are old enough (by which I mean walking and able to do a pincer grip) then have them help you clean up.
Children learn by example, and they see more than we think. If you verbalize what they may not see or understand then it contributes to their learning. They won't be tidy eaters over night but they will pick it up eventually. :)
Just remember you cannot get upset about something they do if they have seen you do it, beware they will notice! Everything you do teaches them what to do.