Spray liberally with WD40 or degreaser until it runs clear out the back. Let dry for around 24 hours (probably longer). Dribble in the oil. Don't go squirting it. A heavier oil is preferred.

Taking apart a freewheel is pretty simple, and can be a fun experience... if you like hunting around for umpteen bearings that fall, spray and bounce everywhere. A magnet on the bench can be a very handy thing.

And you can never find enough of the little blighters to entirely fill the races (I don't think they do anyway in the factory), and you need smears of grease to keep them in place. And by the time you have picked 100 or more little balls out of the solvent and wiped them clean with a rag... well, boring.

Then you need to make sure you don't misplace the very thin spacers that determine the preload on the bearings, and how much the freewheel moves up and down when you're coasting.

Having said that, it's something every aspiring bike mechanic should try... once. Of course, knowing what to do comes in handy when you haven't got the correct fit of tool to remove a recalcitrant freewheel body from the hub using a vice, or if you desperately wish to service the pawls and springs inside.