I used to have my sleeping bag held on by one or two bungee cords, floppin' and unravelin' all over the damn place until some kid showed me how to bungee it up properly.. Pretty simple, just roll it up real tight, wrap a bungee cord around the left side of the rolled up bag and a bungee cord on the right (so they'll be parallel, not too close together but obviously not so far apart that they're slipping off the sleeping bag), then loop a shorter bungee horizontally through the parallel cords, make an X with the short one and hook it onto whatever you can on your pack.
If you have another place to put a bungee cord you can make the bag a lot more secure. Example: mine has a pocket with two vertical buckles, I slide a short bungee horizontally through there, make another X and hook it on to the parallel bungee cords.
Hopefully that makes sense, hard to explain those kinda things over the internet.