Read the Quidco website for full details of how the cashback works. In summary - you go to the Nokia website via a link in Quidco (you have to be registered*). Make your purchase as per normal (with the 10% code). If the purchase "tracks" properly then 20% cashback will be in your Quidco account and after several weeks you can get it paid into your bank.

*Registration is free but Quidco charge/take £5 per year from any cashbacks you make (in that year).