Place the thinly sliced beef in a large resealable plastic bag or shallow dish.
In a medium bowl, whisk together soy sauce, Worcestershire sauce, brown sugar, smoked paprika, garlic powder, onion powder, black pepper, and red pepper flakes until well combined.
Pour the marinade over the beef slices, ensuring all pieces are evenly coated.
Seal the bag or cover the dish and refrigerate for at least 6 hours or overnight to marinate.
Remove the beef from the marinade and pat dry with paper towels to remove excess liquid.
Line the slow cooker insert with aluminum foil and place a wire rack inside the slow cooker to elevate the beef slices.
Arrange the beef slices in a single layer on the wire rack without overlapping.
Cover the slow cooker with the lid slightly ajar to allow moisture to escape (you can prop the lid open with a wooden spoon handle).
Cook on the low setting for 6 hours, or until the beef jerky is dry but still slightly pliable.
Remove the jerky from the slow cooker and let it cool completely on a wire rack.
Store the cooled beef jerky in an airtight container or resealable bag at room temperature for up to 1 week or refrigerate for longer storage.