First, collect some colourful leftover yarn. This is the hardest part as all the colours should be bright and look nice together. Actually, I decided not to bother and use some variegated sock yarn that has been lying around since last winter.
Then spend a few evenings watching good old films and knitting a loooooooooong I-cord.
If you don't know what I-cord is, look here http://www.purlbee.com/i-cord-tutorial/
, or just about anywhere. Don't be scared if you've never touched knitting needles before; it is very easy