Here's how I cleaned mine. (Make sure to watch the videos I posted, they WILL help you a LOT)
1. Take gun apart (completely apart. There are videos on youtube about how to take it apart, and how to put it back together. )
2. Boil water (a lot of water. I kept two pots boiling at all times)
3. Pour water into a pan (make sure it is one you will never cook with again. I used a sturdy foil pan that was about 3" deep (I think it was about 12"x24"))
4. Soak parts in water for about 5-20 minutes (until all the cosmoline has melted off and floated to the top. DON'T let it cool off or the cosmoline will settle back on the parts)
5. Remove parts from water (it is HOT, try not to use your hands).
6. Rinse parts off with hot water (it doesn't have to be boiling hot, but it should be hot enough that it can melt cosmoline) (this removes any cosmoline that was picked up then removing the parts from the water) (also, you can use dish soap just to make sure you get everything off, but I don't know if that is necessary when doing it this way. Just make sure to rinse all of it off if you do use it.)
7. Dry parts off and check for any remaining cosmoline.
8. Repeat as needed. (it is a good idea to replace the boiling water between batches of parts)
9. Oil ALL of the parts.
10. Run a cleaning rod through your bore to clean it. There will be a lot of black residue/crud if your mosin was anything like mine.
11. Put it back together and enjoy. (make sure the bolt is assembled correctly. Here is a video on how to make sure everything is adjusted properly.)
If there is any cosmoline on the ammo pouches, or the sling. How water will also clean them. Just lay them out to dry.
Hope this helps. Have fun, Mosins are cool guns.