Trim any excess fat from the beef flank steak and slice it into thin strips about 1/8 inch thick, cutting against the grain for more tender jerky.
In a large bowl, combine the 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 sliced beef strips to the marinade, ensuring all pieces are fully coated. Cover and refrigerate for at least 4 hours or overnight for best flavor.
Line the slow cooker insert with foil and place a cooling rack or a wire rack inside the slow cooker to keep the beef strips elevated from the bottom.
Arrange the marinated beef strips in a single layer on the rack without overlapping. You may need to do this in batches depending on the size of your slow cooker.
Set the slow cooker to the ‘Low’ setting and cook the beef strips uncovered for 6 to 8 hours. Check after 6 hours; the jerky should be dry and flexible but not brittle.
Once done, remove the beef 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 plastic bag. It can be kept at room temperature for up to 1 week or refrigerated for up to 3 weeks.