Combine all marinade ingredients and marinade beef for 4-12 hours. If you are using a thicker cut than flap meat (i.e. flank steak) you may want to marinade for longer or overnight.
Discard marinade and grill over hot coals or gas bbq.
Slice against the grain (very important!) in to small pieces and serve as part of tacos, or nachos, or anything else!
Notes
In California, Beef "Flap Meat" is very easy to find. In other parts of the country/world it may be called Sirloin Bavette or Bavette.
It's a similar cut to skirt steak or flank steak, in that it has a rich beef flavor and great for grilling and these can be substitutes. All of these cuts have muscle grain that run parallel to each other, and it is very important that when you slice the beef, you are slicing against the grain to ensure tenderness.