In a large bowl, combine soy sauce, Worcestershire sauce, brown sugar, smoked paprika, garlic powder, onion powder, black pepper, and red pepper flakes. Stir until the sugar is dissolved and the marinade is well mixed.
Add the thinly sliced beef to the marinade, making sure each piece is fully coated. Cover the bowl with plastic wrap and refrigerate for at least 4 hours or overnight for best flavor.
Remove the beef slices from the marinade and pat them dry with paper towels to remove excess liquid.
Line the slow cooker insert with a clean kitchen towel or cheesecloth, allowing the edges to hang over the sides to catch drips.
Arrange the beef slices in a single layer on the towel inside the slow cooker, making sure 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.
Set the slow cooker to the low setting and cook the beef for 6 hours, checking occasionally to ensure the jerky is drying evenly.
After 6 hours, check the beef jerky for dryness. It should be dry but still slightly pliable. If needed, cook for an additional 1-2 hours.
Once done, 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.