Basically, this isn't too tough--you replace the cap in reverse of how you removed it. It's important, however, that you get it on tightly. The cap has two wide tabs, opposite each other on the cap, hanging down slightly. Measured across, each tab is about one quarter of the circumference of the cap. The fill spout, where you poured in the coolant, has two notches or cutouts that match up with the tabs of the cap.

• Match the tabs with the cutouts on the fill spout. Press down firmly on the cap and turn it to the right a quarter turn. Give it a jiggle to be sure it's in place. Though the cap shouldn't be tight at this point, you shouldn't be able to lift it off, either.
  
• Turn the cap another quarter turn to the right, still pressing down, until you feel it lock into place. It should be locked tight now and won't move if you try to wiggle it.
  
• With a rag, wipe around the cap to clean off any coolant that might have sputtered out. That coolant smells awful if it's left there to burn off!