For earning extra cashback you just have to follow these steps :
1) Login to one of the cash back site.
2) Find link of your favourite shopping site (easily visible with percentage of cashback provided)
3)Click on link
4) Start your routine shopping.
To get much better results and to save more just screen different cashback sites and find the best cash back offer. As every site provide different cashback offers for particular shopping site.
Below is list of some popular cash back sites: