Place the thinly sliced beef in a large mixing bowl.
In a separate bowl, whisk together soy sauce, Worcestershire sauce, brown sugar, smoked paprika, garlic powder, onion powder, black pepper, and red pepper flakes until the sugar dissolves.
Pour the marinade over the beef slices and toss to coat evenly.
Cover the bowl and refrigerate for at least 4 hours, preferably overnight, to allow the beef to marinate.
Remove the beef from the marinade and pat dry with paper towels to remove excess liquid.
Line the slow cooker insert with foil and place a wire rack inside to keep the beef elevated.
Arrange the beef slices in a single layer on the wire rack, ensuring they do not overlap.
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, checking occasionally to ensure the beef is drying evenly.
After 6 hours, check the jerky for desired dryness. It should be dry but still slightly pliable.
Remove the beef jerky from the slow cooker and let it cool completely before storing.