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 'BigGiftList - Add Item' bookmarklet button.
A new pop-up will appear (you may need to configure you browser to allow the pop-up to display), 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. Your done. The window can be closed and you can continue browsing the web.
If you do not currently have the 'Bookmarks Bar' shown click the 'Opera' menu on the top left of the window, move down to 'Bookmarks' and click 'Show bookmarks bar' in the menu.
In the dialog window that opens you can name the bookmarklet. Once you're happy with the name click Save.
You are now ready to start using the bookmarklet to add items.