You can:

1) Left-click on the overlay to create a task of the type selected in the toolbox. Please note that though you can have multiple Waypoint tasks, only one of each other tasks can be used.

2) Middle-click on a task to delete it.

3) Right-click on a task to modify its properties.

4) Set the global properties for the entire mission by adjusting the slider bars.

4) Click 'Finish' if the mission plan is complete.