Usage:

• Either select an area of the predefined Areas
• Or use "navigate" and left side overlays or mouse to navigate and zoom the map to your desired area (mouse wheel zooms, drag&drop moves the map), then switch to "select" mode and mark the area you want to extract. (click on top left of the area and hold mouse until you reached bottom right corner of your area).
  

On the right side your coordinates and the size of the map will appear (just for informational purposes).
Finally click the "Get map!" Button to Download your Map or "Submit for Predefined Area" to have your selection included in the predefined areas on next release