Go to Window → Map editor. Then go to File → New. Black canvas should be yellow now. It's time to give a name to and save new .th2 file, so relative paths to other things (as background images) can be stablished.
The “Background images” control panel is at the bottom of the right part of map editor window. Click on “Insert” button. If you haven't saved .th2 file yet, a warning message will be displayed asking for it. Press OK, then a “Save” window will appear.
After new .th2 file is saved, another window will appear. This time it asks for image you want to load. Choose the new .xvi one. You don't need to care about other buttons in this panel: they are intended to be used with scanned sketches.
There is a zoom control in “Drawing area” panel, just over the “Background image” panel. Click on rectangle next to “Zoom 100%” to see zoom menu. Also, you can scroll the canvas.
Now you are ready to begin your first scrap.