What to do with the paper tissue

Can one crumple up the paper tissue and leave it on the plate after a meal?

Neither a cloth nor a paper napkin should be left on the plate after a meal. Both should be placed on the table after you have finished eating. If the plate is still in front of you, just put the napkin next to the plate on the left. You can also slide paper tissues a bit under the edge of the plate when sitting on a breezy terrace.

After having folded the napkin, make sure to place it such that the opening points towards you or that the fold points to your neighbour on your left. Even if this means to be perceived to be pedantic. But this is the etiquette. In this way, your neighbours cannot see any food remains.

Most guests agree that cloth napkins should not be placed on leftover gravy. Paper tissues are placed on the plate as a matter of course in many places. The service personnel oftentimes do it, too. And that is fine. It might be somewhat unpleasant when the service personnel have to reach into dirty dishes every time to retrieve the dripping wet paper that the guest has left on the plate. Under no circumstances should the napkin be left on a chair or the back of a chair.