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.
I have left feedback for:
"On a Pale Horse" by leveragehunters (Monkeygreen) (MCU/Avengers AU)
Steve gets turned into a horse.
(Stands alone perfectly even if you don't follow this fandom.)
"Search, Rescue, Remedy?" by dialecticdreamer (Polychrome Heroics: Gentle Soul)
Abioud and Aidan rescue a family stranded in a boat.
(Stands alone reasonably well even if you don't read this fandom.)
"Mina" (Part 1 and Part 2) by siliconshaman (original work)
Explorers discover and old ship frozen in ice ... and something unexpected.
(I know, this one isn't fanwork, but when I looked at the author's recent fanfics, they're all heavy in continuity. If you're reading Polychrome Heroics: Berettaflies, though, there are new-ish goodies up.)
"Older Than I Once Was" by Samalander (Wonder Woman)
An exceptionally well-written Blackfoot perspective on the events of World War I.
"Oil and Watercolor" by Dedicatedfollower467 (Batman)
This is a soulmate story using the motif of people leaving colored marks on each other's skin -- and it's the first I've read where the intensity of the color indicates the amount of impact on a person's life. Fascinating.
Also, I wrote these for my Crowdfunding section, but they generalize well to fanfic:
"How to Boost Your Audience"
"How Make Constructive Comments"
"How to Support Your Favorite Author"
"Non-cash Support Methods for Crowdfunding"
"People Who Boost the Signal in Crowdfunding"
"When You Can't Find an Audience"