After skating through water and dirt:
Use the truck to pry the bearings off the wheel so as not to damage them, then carefully remove the end plates without bending them.
Place the bearings in a container and add cleaning solution (acetone or an oil dissolving solvent).
Dry the bearings on a suitable surface.
Once dry, lubricate them with oil.
Finally, replace the end plates and ensure they're in place before mounting the bearings on the wheels.