Great question.
After you've installed the chrome extension or safari extension, get logged in and go to your favorite webpage. Alternatively, the Are.na bookmarklet lets you save web pages and media directly to a channel without visiting Are.na.
Save the entire webpage
Save text by right-clicking on a selection
Save an image by right-clicking on it
Save an image by dragging it into the window
Edit a block before you save it
Create a new private channel