Select "Polygon" mode from the toolbar. Choose a layer from the layer panel in which to create a new polygon. Left-click at the first vertex of the polygon. Move the mouse to the next vertex and place a new one with a left mouse button click. Move to the next vertex. Depending on the connection mode, the edges created are confined to certain directions. See Basic Editor Options for a detailed description of the modes. Use the "editor options" dialog (F3 shortcut) to change the mode, even during editing.
Double-click at the final point to finish the polygon. Press the ESC key to cancel the operation.
A polygon will never be "open": there are always edges connecting the current vertex with the initial one. Depending on the mode, this final connection is either a straight line or a combination of edges. In "diagonal mode", there are manifold possibilities to create a final connection in a more or less smart way. The program uses some heuristics to determine one feasible combination. Although this heuristics is not infinite smart, it should be easy to lead the algorithm to the desired solution, by pointing the mouse into the desired direction.