If your bookmark bar is hidden press Ctrl(PC)/Cmd(Mac)+Shift+B to show it.
To begin using cast.li just drag the bookmarklet to your bookmark bar.
When you have the bookmarklet you can go to a website and activate the bookmarklet. Then the cast.li bar will open up and you are ready.
When you have the cast.li bar open you can start highlighting text and when you are done you can save the cast and share the link or save it for later use.