My bin is roughly a 36 inch cube with the posts extending into the soil for stability. You can easily change the dimensions to fit your needs.
What you will need:
(15) 1 x 4 inch boards, 36 inches long. These are the horizontal boards.
(4) 2 x 2 inch boards, 42 inches long. These are the vertical posts.
(60) 1 1/2 inch galvanized nails.
Drill with small bit to drill pilot holes.
Did you know that most large hardware stores like Lowes and Home Depot will cut wood for free? I go during the slower weekday hours and it is so convenient. I bought 8 ft untreated pine furring strips (that's the cheap wood), and had them cut at the store. So easy.
Here's a tip to make sure you are getting wood that is not warped. Hold it in your hand with the board tipped on its side with the other end resting on the floor in front of you. Look toward the end that's on the floor. You can tell if the wood is straight or not. Pick the straightest boards.
To prevent the boards from splitting it is helpful to drill pilot holes in the boards where the nails will be hammered. Drill 2 holes in each end of the 1 x 4's. Pilot holes also help to make the nails go in easier and straighter for those of us who have some trouble with a hammer. Can't it be frustrating?
I designed this bin so that in the future if I decide to make a cover there will be a one inch gap between the cover and the top board. This will help with ventilation. That's why you see the posts extending higher than the top boards.
Mark one inch from the top of the 2 x 2's.
On a flat surface arrange 2 vertical posts then align the 1 x 4 board with the mark on the 2 x 2. Use a carpenter's square to be sure it is square.
Nail the board to the 2 posts. This is the top board.
Mark one inch down on the vertical post. This will leave a one inch gap between the boards for ventilation.
Nail another board using the carpenter's square to be sure it is aligned correctly. Continue until you have nailed 5 boards. This completes one side. You will make 2 of them. There will be 5 boards remaining.
The next part required an assistant to hold the 2 side parts on their sides. Lay a board across the top, aligning it with the other boards. Again use the square before you nail the boards to the posts. You end up with this.
Level off the ground and place the bin where you chose. The posts will make a mark on the ground where holes need to be dug for the posts. Use a level to be sure the bin is level.
Use the carpenter's square to make it square.
Fill the holes, then fill your compost bin!