How can I wire this RF receiver up to control a low-voltage motor?
Actually the diagram is self-explanatory but however you should do the following:

connect


the + of your power supply to IN+
the - of your power supply to IN-
IN- to COM
one motor terminal to IN+
the other motor terminal to COM


the diagram says that the supply could be 6v, 12v or 24v so choose the one which is the same of your motor's voltage.