Trim silver skin and icky fatty parts from your flap meat and cut them into 3 or 4-ounce chunks, or whatever size you like.
Put them into a container with a tight fitting lid; don't forget to leave extra space for the marinade.
In a large bowl whisk together the Worcestershire sauce, soy sauce, molasses, brown sugar, pepper, pepper flakes, and garlic.
Add the stout- careful, it will foam a bit.
There is really no need to add salt; the soy and Worcestershire are plenty salty.
Pour the marinade over the tips and gently stir to coat evenly, if you don't use all the marinade it will keep for 2 weeks, refrigerated in an airtight container.
Refrigerate the meat for 8 hours or overnight.
(Will keep, refrigerated for 4 or 5 days) and then cook as desired.