If you have any bits of wool either left over from other projects, or from things you've picked up a top shops - this is what I do with them - it's so easy you could do it falling asleep. First of all do either 20, 30, 40 or 50 chains, depending on how big you want your bag to be. Then crochet 3-5 double crochets, then a treble for the cord to go through. You simply continue on using double crochet, attaching new pieces as you run out of your old ones. They can be as long, or as short as you like, and you continue until you have a length that suits you. I usually measure up as I get towards the end. Then do your treble, your 3-5 double rows, crochet the sides together. I usually go all the way around, as it creates a border that is the same color as the top of one side, which finishes it cleanly. (Though I have had instances when that color has run out, and I've just tied on another one.) When you've crocheted the seams, just do a long row of chain, I usually go for 150-200 chains, then, depending on how thick you want it, stop it then, or go back to create a double rowed cord.
Then it's just a matter of threading the cord through the treble row you've done. I usually leave all the tied bits out, giving them a really Ratty look. Of course you can weave all the threads back in. It's entirely up to you what kind of effect you want.

A bag will take 2-3 hours at most, and it's a great way to recycle old bits of wool, and to relax on the train or bus on your travels.