After installing the extension, navigate to IG's website in a new tab and log in. From there you have the following options:
Download regular IG posts images/videos:
-click on a link to a post anywhere IG to open it.
-a confirm dialog should pop up asking if you want to download the files.
Download IG stories:
-click on a story to open it in the story viewer.
-click start download button on top left of page and stories should start downloading as they are played.
-click stop download button on top left of page to stop downloading.