Take everything apart that comes apart. Throw parts in a bucket with some dish soap. Dump a pot of boiling water over it and wait a minute. Tilt the bucket back and forth a few times to give the melted cosmo a chance to come out and float to the surface. Gently pour water out, being careful not to dump important parts down the drain. (Don't put plastic or wood parts in boiling water...clean them separately).
Repeat.
This will get rid of most of the cosmoline, but you'll still have some residue. Clean each part with dish soap and a toothbrush. Use a pipe cleaner, or something similar to clean small holes, like the firing pin channel. Rinse with hot water and put each part on a towel or newspaper as you clean it.
Oil everything with gun oil. CLP spray or something similar will be the easiest way to make sure everything is coated. Wipe off excess with a rag. Run a patch down the barrel to remove excess oil.
Put it all together and give it a final wipe with a clean rag. You're done...