So easy I don't know why more people don't do it.Get any homebrew beer kit. Or lager, mild, old ale anything!
Go to the hops page on beer accessories and buy a pack of hops, they're in packs of about 110g or 4oz for those who haven't been metricated.
Which hops? There's hundreds on there? Well, any! As a rough guide I'd use Fuggles for mild, Goldings for bitter and Saaz for lager.
Open the pack and break off about a quarter of the pack, thats about 25g, it oesn't have to be precise.
Put them in a jug and pour over a pint of boiling water, give them a good stir and leave to cool.
Strain through a small sieve or a piece of net curtain and add the liquid only to your beer!
You can add it to the bucket when the yeast goes in or just before you barrel/bottle.
Try it!! If you'd like it more bitter add a few extra hops next time.
Even easier just buy some hope extract and add it! Needs no boiling - It's on the hop page!