Here, we’ve added a bunch of blood plasma molecules around the virus. If you make a mistake, chose the eraser option at the bottom and click on molecules to remove them. You can also shift-click to remove all copies of a particular molecule.
Two other drawing options will help you refine your scene. The Nudge icon (a little hand) allows you to click a molecule and move it. The Pin icon lets you pin a molecule in one place, so it won’t move.