How do you properly cook cow tongue?
Standard prep for beef tongue seems to be: scrub it well and rinse, then soak it in cold water for 2-3 hours.  Heat water to a low simmer (about 200F) and cook slowly for at least an hour per pound.  Preferably cook as long as you can (5-6 hours) to get a more tender result.  When meat is tender and cooked, remove meat and save the broth. Cool the tongue and peel off the outer skin.  The broth may be reduced and added to vegetables and meat to create a richer, meatier taco.  

I also found a tongue taco (how is that NOT slang for some sexual act?) recipe.

Personally, I'm also a big fan of cold boiled beef tongue with horseradish sauce.  You can go from the cooked tongue straight to that just by slicing it up. Now I really want to make tongue tacos.