Usually, there is no code that you have to put in... You just have to follow the link that Quidco provide on their site (which installs a tracking cookie) then you just buy from the web store as usual, and the web store will read the cashback information from the cookie.
It's usually advisable to clear your cookies first, in case there is overlap between different cookies for the same store that may confuse it. Then, you have to find the 3 shop page on Quidco, click on the link, then buy the phone as you normally would.