First you need to open the drawing document that is going to be exported, then you need to select the objects that you want to export. Normally, you can select these objects by one or by opening a window which contains all the objects and export them all. Once these objects are selected, you need to access File > Export > File and select the type of the file (DXF, in your case) select a name and that's it. There are no hidden or complicated export procedures.
