Sign up to Quidco here.
Once you've registered, log into your account before ordering anything online and click the link on their site to the site you want to order from (note: not all sites are on quidco, but it's good to check before hand, just in case it is there). After clicking the quidco link to the site you want to order from, complete the transaction as usual.
Money you've earnt can be tracked on your quidco profile and can be withdrawn from there as well. The amount of cashback you earn varies from site to site, but can range from a percentage of your shopping back to lump sums for ordering specific things, for example £35 for taking out a pay monthly contract with O2 (correct at time of writing).
You can withdraw your money by PayPal or BACS (PayPal up to £50 at a time). To withdraw your money you need to set up a minimum payout that is paid out to you each month, providing you have received enough for this to be paid (this includes ordering, having cashback tracked, validated and received into your account). Once this is done your money will automatically be paid out to you in the method you require, each month, providing there's enough money to do so.
Once you have specified a payment method, the minimum payout will be £0.00 meaning you will be paid every month, regardless of how much. I recommend leaving your money to build up to pay minimal PayPal fees (I have yet to take any money out of quidco, at time of writing). The only charge for using quidco is a £5 annual admin fee. However, this is taken out of the first £5 you earn through Quidco, so you don't need to pay anything to Quidco at all, and if you don't earn anything in a year from Quidco, you don't pay anything.
Use a different browser for Quidco and delete all the cookies beforehand to be sure that your purchase will track. Using a browser exclusively for Quidco will preserve your daily internet habits.
Once you've submitted a product, Flubit will email you if it's able to beat the price. Flubit will sell you the product from a different retailer at the lower price. Sometimes the price won't be beaten but you'll get a response either way within 48 hours. From time to time you'll somehow get a cheaper price for a product on Amazon and it will be sent from Amazon itself. When something like this happens, rest assured that a wizard did it.