Some guests in trying to be nice and considerate will carefully fold their napkins as if they are making it easier for them to go back to the linen closet or wherever they live in that particular household or restaurant. This isn't necessary or expected. Just leave it next to your plate, not in a ball but just in a loose, narrow and neat - somewhere between a cone and a triangle, the easier to throw into the washing machine. In Italy when someone folds their napkin it's seen as a sign that you'll never come back to that house again. They don't like that especially if they like you.
Another nicety guests will offer is to stack their plates, again trying to make clean up easier. Maybe if you're at a super casual cookout or picnic, the plates are paper and if you're volunteering to help this is fine. When you're at a dinner table or in a restaurant stacking actually makes clearing the table more complicated.
Your fork and knife should be placed at 5 o'clock, next to each other on the plate. This is universal. Wherever you go in the world they'll know that you're finished with your meal and your plate is ready to be removed.
A friend of ours hates to see as he puts it, 'spent food on a plate.' The host is probably waiting for everyone to finish and it would be rude to start clearing the table any sooner. If everyone is finished, there's nothing wrong with offering to help. There is no need to say how much you hate looking at bits and pieces of leftover dinner on the plates.
It's not up to just the guests to do the right thing at the end of the meal, hosts have their jobs to do too. One thing we never do when we give dinners at our apartments is to disappear into the kitchen and clean up everything. We think it's unacceptable to get a head start on cleaning while you still have company. Stack the dishes in the sink or on a counter and go back to your guests. They didn't come to hear the rumble of your dishwasher. If you're just hanging out with good friends or close family, that's another story. But that usually isn't the story.
The next time you're invited to dinner, have a good time, you're the guest.