Clear everything from the mattress before folding it up. This includes all pillows and blankets that may have been used.
Fold the lower third portion of the mattress over so that it sits on top of the middle portion.
Lift the part you just folded, and fold that over into the couch frame. There may or may not be a lever or handle that you can grab for assistance with pushing the mattress down.
Make sure the mattress is completely tucked away inside the couch frame. It should be flat, with no parts of the frame sticking up or out.
Replace the couch cushions and add any decorative pillows to your sofa. It is now ready to use for sitting.