Safety Tip: Always wear safety glasses when working with tools that could make something “fly” around. That actually means anytime your are using tools whether hand or power. You only have one set of eyes, protect them.
Next cut 5 pieces of 2×2, 36 inches long. Be sure to clamp the 2×2 to the box before you cut. This is much easier than holding it and much safer even if you are using a handsaw. These will be cross supports for the box. Next we will cut the legs. Treated wood should be handled with gloves whenever possible. Cut 8, 36″ pieces from the treated 2x6x12 that you purchased. Use the clamp to secure the piece before you cut. It’s like having an extra hand. Cut these pieces individually or you might end up with some shorter than others. The last piece will need to have 1/4″ to 1/2″ cut off the end.
On the box put a mark about 3/4 inch from the edge and 53 inches from the end. Make two more marks 26.5 inches from the end and two more 2.25 inches from the end. Drill 11/64 pilot holes 3/4 inch form the edge on your marks. Start 3 inch screws in each hole. Use the 3 foot 2×2 you just cut and center one under each hole making sure the 2×2 is flush with what will now be the bottom of the box. Once centered run in the screw and repeat with the other four. Turn the box onto the other side and repeat the procedure for the 2×2 securing them at both ends. The edge you attached the 2×2 supports to is now the bottom of the box. Leave the box standing on it’s side. It will make a great table for the next step and it’s easier to attach the legs.
Now we will build and install the legs. Measure and mark the center about 3/4″ from one edge and then about 3″ from each end on the same edge. Drill 11/64 pilot holes at your marks. Repeat this with 3 other leg pieces. Start 3″ screws in each of the holes you just drilled. Now take one of the un-drilled pieces setting is flat on the side of the box. Put one of the screw started pieces on edge next to it with the screws lined up so they will go into the edge of the flat piece. Be sure to line up the tops and bottoms of the pieces and then drill the screws all the way into the boards. Repeat this for the other three legs. Now attach the legs to the box. Be sure the legs extend toward the bottom of the box. Drill one pilot hole in the side of the leg you will attach first. Start a 3″ screw then lay the leg on the box where it will be positioned. Be sure to make it flush with the top of the box. Once the leg is properly positioned run the screw all the way into the leg and the side of the box. You could use the clamp to hold the leg in place. When is played out on the corner it is quite heavy to hold. Drill two more pilot hoes on the side with screw, use a triangular pattern for the holes. Drill three in the side not yet attached. Start the screws. Run them home. Do the same for the remaining legs. Set the box up on its legs.
and many, many others.