1) Join the cashback site
2) Shop at any of the 4,000+ online merchants it supports
3) Earn money on each purchase, either as a % of spend, or as a fixed bonus amount
4) Withdraw the cash to your bank account, or receive it as shopping vouchers
Alternatively, you can pay £5 a year to get a Plus account that will earn you higher levels of cashback, and give you other bonuses too. (The £5 annual fee will be deducted from the first cashback you earn.)
topcashback.co.uk/ref/newmembers…
(If you use the above link, I get a small bonus at no cost to you after you start earning cashback. Thanks so much! Or Google "topcashback" to go direct.)