I just wipe them off a clean as possible then take a blob of grease and work it by hand all over the outside of the cartridges kneading it into what ever openings there are. There is no systematic way I know of.

BTW, new cartridges are available if yours get too worn. They are nearly the same price as a new headset but avoid having to remove and press in new races. Alternatively but the complete headset and just use the bearing cartridges.