To lanolize wool, first take some lanolin (such as lansinoh) and squirt about a half inch of it into a mug. Fill the rest of the mug with water and add a squirt of baby soap. Micrwave it for 1-3 minutes, or until the lanolin is melted. Stir well. Add this mixture to a bowl of cool (not cold) water. Add your cover and swish around, turn the cover inside out and swish some more. Let the cover soak for a few hours, or overnight.
I found that if you use a high quality wool wash or wool soap bar such as WOW wash or a bar from The Soap Deck you don't need to do the lanolin thing. The washes contain lanolin themsevles.
I wash my wool whenever it gets solied or if it starts to smell a little ripe