Step 4: A Brief Overview of Ned's Object Editing Capabilities
Move the mouse around the level, and click to place objects at the cursor's position (specifically, beneath the centre of the 'x'). Some objects, such as most enemies, require more than one step before they can be placed. Follow the instructions displayed around the cursor.
Though there is no 'Undo', you can delete individual objects by moving the cursor near or on them, holding 'backspace' and clicking. You can delete objects in reverse order of their creation by holding '\' and clicking.
See the images attached to this step for a quick look at some of the features of the Object Editor.