Repeat after me: WD-40 is not a lubricant!

But you CAN use it as a part of cleaning them, if you don't want to spend the money buying new ones.
Honestly, if the bearings are locked up, just throw them. If they spin very rough, consider throwing them. If they spin but just not for long and are rough, there may be hope. Consider time/materials vs new cost. You will not make them feel like new ones in most cases, but you can salvage some of the bearings if you pick your battles

Here's what I did:
1-Soak them in WD-40 to help remove the old grease/dirt/oil
2- Soak them in CLR (Note: this is the most dangerous step as too long and it will eat away at the metal since it's corrosive, too little and you won't get enough rust off)
3-Rinse in water, or ideally 90%+ isopropal alcohol, to prevent rusting.
4-Soak them more in WD-40 to remove the last of the CLR and other marterials
5-Rinse in 90%+ isopropal alcohol, dry them fully
6-Lubricate with a very fine oil. 3-In-1 oil generally is acceptable but only use a TINY amount, otherwise you will have oil everywhere.
Note: Between each step, take the bearing and put it in a drill and spin it for a long time to help work out the rust and whatever gunk. Be carefull doing this. A battery operated drill wont have enough torque/speed to do much good, and AC powered drills can be damaging if done too long or too fast, but work better, but of course dangerous to hold the bearing while drilling. If you can drill with the bearing in the liquid(s), it's better to help clean it out, but watch out for splatter.
Also note: If you have sealed bearings (most are), you can either try to remove the cover or just try to clean with the cover on, but you wont get much out of it without removing the cover. Removing it you may bend it and may not get it back on.
Good luck!
PS: If you need to clean the ivory balls... place them in highly concentrated Deck Wash (chlorine bleach).