Once installed your browser will have a new button on it's toolbar. To add items to your lists simply browse the internet for the items you want (e.g. Amazon product page), and then click the Big Gift List button, the one that looks like a present/gift box.
A pop-up will appear; if you aren't currently logged in to your account you will be asked to do so. Once logged in you will be able to fill in the details for the item and select the list you want to add it to. You're done.
You are now ready to start using the Add-on to add items.