I know there are expensive and fancy compression sacks out there specifically designed to act as a waterproof pack for your sleeping bag- but this trick is just as effective and likely won’t cost you a dime. In this demonstration I use a regular compression sack (Outdoor Research), my Mountain Equipment Co-op Merlin -3 down sleeping bag, and a regular ol’ garbage bag.
Most folks intuitively pack their bedding and clothing in garbage bags to keep it all dry, but it doesn’t take long for the unprotected garbage bags to suffer enough perforations to be as watertight as a colander. To prevent this from happening, just line your regular sleeping bag stuff sack with the garbage bag, and then stuff your sleeping bag inside. With the mouth of the garbage bag in your hand, you can use your body weight to evacuate most of the air out of your sleeping bag, and with a few twists and a tuck, voila, you have a compressed sleeping bag, with a watertight barrier around it. Finally, just cinch down your compression sack highly around the garbage bag, so it will act as a first layer of defence from pointy or sharp objects ensuring your garbage bag stays intact.
Thats it. Simple.