How do I respool a grass trimmer?

Hi, I've been looking to get a replacement line spool for my grass trimmer, a JCB LT30500. It seems Pulse Home Products (www.jcb-powerproducts.co.uk) will sell me one for over £7, but somebody on the site suggested buying 5 metres of ANY trimmer line and respooling it myself. Sounds great, but I was just wondering how the devil you do it?! Any help much appreciated! Cheers.