Tap the thumbnail for your story (it will be at the top of your feed). Once you select it, you'll be taken to a screen with an icon on the right top with three dots. Tap it.
Select Copy Link, tap the small button on the screen.
Copy its URL then open the FastDl webpage.
Paste the copied URL onto the indicated field on FastDl.
Tap Download, when you're done choosing what story you want to download in your download!