1. Begin by trimming your tri-tip of any large fat pockets, silver skin, or hanging pieces of beef.
2. Spread a thin layer of yellow mustard on all sides of the tri-tip. This will act as a binder and help the rub stick to the meat. Additionally (And maybe more importantly) It will help to build up the bark on the tri-tip.
3. Liberally season all sides of the tri-tip, with the Grim Reaper Rub. This rub is designed to bring a lot of flavor and help to build the bark on your tri-tip. As you season, focus on keeping the rub layered evenly and getting consistent coverage.
4. Place the tri-tip onto a preheated smoker at 225F and add your favorite smoking wood. Beef can take a decent hit of smoke, so just about any smoking wood will work. Oak and Hickory are common. Add a water pan between the tri-tip and the heat source.
5. Allow the tri-tip to smoke directly on the grills, as far from the heat source as possible. After the first two hours, begin spritzing all visible sides of your tri-tip with the beef stock every 60 minutes.
6. Once your tri-tip hits about 160F, roll out a 2x2 foot piece of butcher's paper. place the tallow in the center, followed by the Tri-tip. Wrap it up tightly and return it back to the smoker so that the side of the tri-tip which was facing up, is now facing down.
7. Smoke the tri-tip until a thermometer inserted into the thickest part of the beef hits 195F.
8. Remove the tri-tip from the wrapping and return back to the smoker, with the original side up. Allow the tri-tip to smoke uncovered until the internal temperature hits 200F.
9. Remove the tri-tip from the cooker and tent lightly in foil for 45 minutes before slicing against the grain and serving.
10. Remember, the grain of the tri-tip runs in different directions across the muscle, so it is critical to pay attention to how you are slicing. Slice into 1/4 inch thick slices and serve immediately.