As I already stated in the link on Square Foot Gardening, I chose to use Pressure Treated Wood for my planters. The basic planter is a 2 foot by 8 foot box. I chose these dimension because I could build each planter using only 2-1/2 boards per planter or two planters using exactly 6, 8 foot by 6 inch lumber.
There are 4 ways that you can build the boxes depending on which boards you attach inside which boards. It may not seem like to big of a difference but the way you construct the boxes will determine the area of square inches you finish with. For instance if you place the 8 foot side boards inside the 2 foot end caps you will wind up with a inside dimension of 96 inches by 21 inches. This will leave you with a inside useable planting area of 2016 square inches because you will loose the 3 inches (1.5 inches X 2 board ) of width down the entire 8 foot length of the box
If you place the 2 foot end cap boards inside the 8 foot side pieces then you will only loose 3 inches down the 2 foot end caps. and will have a inside area of 93 inches by 24 inches and this design will leave you with 2232 square inches of useable planting area.
The 3rd way to build the box is to alternate every other corner so that the first corner has the 2 foot end cap inside the 8 foot side board and on the next corner put end cap on the outside of the side board. This will give you a inside area of 94-1/2 inches by 22-1/2 inches and leave you with useable planting area of 2126 1/4 square inches. If you wish to build a 4 foot by 4 foot square planter this is the best way because your outside dimension will be a perfect square but on a rectangle planter it will not give you the most inside planting area
If money is no object you course choose to use 8 foot boards and 2 foot 3 inch boards but you will be wasting 15 inches per 8 foot board. and you will need 7 boards to build 2 planters.
I chose to use the second method for the most inside area. This method also makes layout easier because each box is exactly 2 feet deep.
Because I'm a bit paranoid and I was using Pressure Treated Wood, before I assembled the boxes I chose to wrap the top, bottom & inside of each board in thick sheet plastic to prevent any of the copper from leaching into the soil. After I wrapped the plastic and stapled it in place, I assembled the boxes using 3 in. Tan Polymer-Plated Flat-Head Star Wood Screws (They are easer to screw into thick wood then Philip head screws). I live in the city but it is country enough that I still have to worry about moles, field mice, voles and other critters so I also chose to attach a layer of chicken wire to the bottom of each planter as well as a layer of landscape cloth to prevent weeds from growing up into the planters.