Choose a building material. Wood is the best choice for the average layman due to its low cost and ease of use. Other materials such as steel would work as well as, if not better than, wood, but are not practical due to expense and difficulty of construction. Wood is relatively lightweight and easy to use, and will last a lifetime if taken care of properly.
Cut all wood to length. You will need two 2-by-8 sections one foot long, two 2-by-8 sections three feet long, four 2-by-4 sections four feet long, eight 2-by-4 sections 15 inches long, four 2-by-4 sections 1 ½ feet long and two 2-by-2 sections 1 ½ feet long.
Lay the one-foot sections of 2-by-8 next to each other to create a piece 16 inches wide by one foot long. Screw one of the 15-inch-long 2-by-4s vertically to both ends of the boards to attach them together. Do the same with the three-foot sections of 2-by-8. You should now have two 16-inch-wide sections of board that will serve as the laying platform of the bench. Attach two metal C brackets 12 inches apart, three inches from one end of the three-foot section. These will hold the 2-by-2s onto this section of the bench.
Cut one-inch-deep notches at six, nine, 12 and 15 inches from the ends of two of the four-foot-long 2-by-4s. Lay the two four-foot sections of 2-by-4 vertically, 16 inches apart, running parallel to each other. Place two sections of 15-inch 2-by-4s inside the long boards at either end, forming a 90-degree angle to the long boards. Screw securely in place. Attach the 1 ½-foot-long 2-by-4s at a 90-degree angle to the four-foot boards on all four corners of the rectangle. These will be the legs of the bench.
Attach the 1-foot-by-16-inch section of wood to one end of the four-foot rectangle you built in step 4. The smooth wood side should face up, with the connecting 2-by-4s sitting in between the four-foot 2-by-4s of the rectangle base. Secure the pieces together with screws. Lay the similar three-foot section on top of the rectangle base in the same manner. Attach the three-foot section to the one foot-section via hinges on the top of the wood facing you so that the three-foot section can be raised and lowered onto the base.
Drill 3/8-inch holes through the ends of the 2-by-2s. Lay both 2-by-2s on the ground parallel to each other, with the holes facing each other. Insert two 3/8-inch-by-17-inch bolts through the holes in the 2-by-2s and the metal C brackets on the backboard. You will need a nut on either side of each 2-by-2 to keep the pieces 16 inches apart. The lower bolt of this piece will fit into the notches cut in the base 2-by-4s, allowing the angle of the backboard to be adjusted.
Attach three strong brackets at a 90-degree angle to each of the last 2-by-4s. These brackets should be placed six inches apart from each other, starting at the top of the 2-by-4s. These will be used to hold a barbell. Attach the last four-foot sections of 2-by-4 to the legs on either side of the backboard with the brackets up. These will serve as the holding arms for a barbell.