Karennina, its dead easy, if you want to order something online go to the 'topcash' site search for that company if they are on topcash it will tell you what % discount you will get. Then you click on the link to that site from the topcash website and order your stuff as usual. then after a while usually 1 - 3 months you get your cash sent either to you by cheque or paid direct to your account.
The big cashback often comes from insurance (car, house etc) and utilities. I have just got £70 for changing from British gas to Eon as well as saving about £230 on my bill. An alternative to topcash is 'Quidco' but they charge you £5 per year out of your earnings.