Lay two 120-inch timbers flat, parallel and with their outside edges 72 inches apart. Position the 72-inch timber against a pair of ends. The ends of the 72-inch timber should be flush with the sides of the 120-inch timber.
Drill four 1/2-inch holes through the 72-inch timber so two holes enter each 120-inch timber. The holes should be 3 inches apart for each pair and should not be closer than 2 inches to the sides of the timber. The 1/2-inch holes should enter the 120-inch timbers and be no less than 3 inches deep.
Drill a 3-inch-deep 1-inch hole in each of the holes on the 72-inch timber. These holes will hide the heads of your lag bolts. Use your ratchet wrench to drive the lag bolts through the 72-inch timber and into the 120-inch timbers.
Drill a ½-inch hole through the center of the 72-inch timber so it is 36 inches away from each end and 3 inches away from each edge. The hole should be parallel with the 120-inch timbers. Insert your eye bolt through the hole so the eye is between the timbers. Screw the nut around the bolt so it's secured.
Dig two 12-by-24-inch holes with their centers 66 inches apart. Set the 120-inch timbers in the holes so they are centered in the holes. Pour 2 cubic feet of cement mix around each timber. Use your level to ensure that the stand is vertical. Pound two landscaping stakes around the timbers at a 45-degree angle so they stay vertical while the cement cures. The tops of the stakes should be flush with the ground. Let the cement cure according to the instructions.
Mount your heavy bag to the eye hook using the hanging hardware provided with the heavy bag.