Handy links to the most common methods you will likely use as a beff outfit creator:

Save Adversary
How to save items from adversary jobs (PS4 only)
Video courtesy of Lenny & Tuna

Save Versus Outfits
Open interaction menu, spam illuminated clothing for 30-60 seconds, leave interaction menu open, get kicked for idling.

Unlock Impotent rage
Item locations to unlock Impotent Rage outfit
Video courtesy of GTA Series Videos

Unlock alien
Item locations to unlock Space Interloper suit
Video courtesy of GTA Series Videos

Flippers
How to glitch the scubasuit shoes into flippers
Video courtesy of GRAVESIGHT

Yellow Rebreather
How to get the yellow rebreather + duffel bag
Video courtesy of Mad Glitchers