Leave feedback for a fanwork. Or multiple fanworks. It can be as simple as I liked this to a detailed list of all the things you loved about the fanwork. The key is to leave some sort of feedback.
If you've already left feedback in the course of a previous challenge, it totally counts. But you're free to leave more feedback.
Afterwards, leave a comment in this post with the equivalent of "I did it!" If you feel so inclined, leave a link to the fanwork you left feedback for so others can check it out.
Timing could not have been better for this one. I've been hanging onto links for a couple stories by fluffernutter8 for way too long and this prompted me to finally go leave reviews for them. Yup, she's still writing and even recently made an LJ post!
Fluffy's stories are here ("The How and the Why", Harry Potter) and here ("Our Inner Wars", B/A for IWRY).
I also started working through a list of GotG recs thoughtfully provided by egelantier on my Day 2 wishlist.