On the page you wish to bookmark, click on the tag button in your toolbar (or select Save a new bookmark from the Delicious menu, or hit Ctrl+d)
If you’re not signed into your Delicious account, a pop-up window will prompt you (click to enlarge):
In the Save bookmark window that appears, you can add notes and tags to your bookmark to make it easier to find later (click to enlarge):
You may see a list of recommended tags based on what you’ve previously tagged, and also popular tags, based on what other people have tagged. Clicking on any of these will add them to your tags:
You can also choose to keep the bookmark private. By default anyone can find and browse your bookmarks (and you theirs.)
Once you’re done, click Save.