https://www.gamespot.com/articles/skull-trooper-fortnites-popular-skin-hits-the-item/1100-6462417/

Update: Epic has recognized that there was some confusion surrounding Fortnite's new Back Bling challenges. In a post on Reddit, it explained that some changes will be forthcoming in the 6.10 update launching next week, adding, "The goal with the Ghost Portal challenges was to provide you with a way to earn a cool Back Bling through your normal play."

In the meantime, a Skull Trooper event has kicked off that makes the new cosmetic easier to get. From now until Sunday, October 14, at 8:59 PM PT / 11:59 PM ET (4:59 AM BST), you'll be able to earn the Back Bling by playing in 20 matches. However, you'll need to own or buy the Skull Trooper outfit on the item shop to qualify for this event. If you go this route, the challenges will complete automatically and award you the Back Bling next week, presumably once the patch arrives. The original story follows.