That’s very simple. Click File -> New.
If you want a background, or change the default width/height of the level, click the Show level settings-button.
File -> Save/Save as.
Click Insert from the menubar and select the object you want.
Simple drag ‘n’ drop them.
Right-Click on the object, and select that what you want.
Right-Click on the object/enemy, and select delete.
Insert a box and change the “BoxType” - setting in the propertys-table.
Insert it and change the “NextLevel” property to the filename (relative to the Data/Levels directory) of the next level (or leave it blank when this is the last level).