Well, of all the muscle groups on a beef, the heart has got to be the toughest. I'm don't know if your beef heart is whole or if it's been cut in half, so we'll take it from the whole heart stand point. Cut the heart in hald from top to bottom. Remove any heart valves that you see. Remove any of the larger veins that you find on the exterior of the heart. The fat that is found around the top of the heart can be trimmed off or left alone-your call. Soak the heart in a brine for 5-6 hours. Rinse the heart pieces off and pat dry, season it up with some cracked black pepper, onion powder and garlic powder. Since you did pull this out of a brine, go lightly with the salt if you use any at all.
Place the heart into a foil pan and place it into your preheated smoker at 250° and smoke for 2-3 hours. Add 1/4-1/2 beef stock to the foil pan and cover with aluminum foil. Cook for an other 2-3 hours. After 2 hours, check the doneness of the heart by doing the probe test. Insert the probe into the thickest part of the heart; if the probe slides right in and out with no resistance then the heart is done. I've had hearts take up to 4 hours before it would pass the probe test (most likely due to the age of the beef). Eat it as it is, or cut the heart into cubes, sprinkle on some bbq rub and put back into the smoker for a 'burnt end' style of heart.