Hi there!
You can find the explanation of how to go about it on the Help section in Finds (at the top of the page):
So first you have to install the bookmarklet by dragging it into the bookmarks bar of your browser. Once that's done, you can start collecting from all the stores that are currently supported (see the list on the right-hand side of that same page). ETA: You have to be logged into your YLF account, otherwise it won't work.
I hope this helps?
Happy collecting!-)
Inge
P.S. It's always so fun to see your name pop up on the forum. Hope all is well on your side?