It's simple. Here's the basics.

Pick an area with 6-8 hours of sun each day.

Just build boxes with 1x6 or 2x6 lumber for the sides. Don't use pressure treated wood, since the chemicals could get into the soil. Cedar is good as it naturally withstands weather well. Normally 4 feet by 4 feet is good as lumber usually comes in 8 foot lengths. Lumberyards will usually cut this down if you can't carry 8 foot lengths in your car or just don't want to do the cutting. Use deck screws to fasten the boards at the corners. If smaller children are going to be tending the gardens, 3 foot by 3 foot might be better. Little arms might have a hard time with 4 foot size frames. You don't want to walk on the soil, but reach across to the plants. Smaller sizes might be better for a deck, such as 2 foot by 2 foot.

Fill the frames with the special mix; 1/3 peat moss, 1/3 coarse vermiculite, and 1/3 compost. All of these are readily available at home and garden centers. This is a big part of why this system is so successful. As hard as you try to improve the crummy soil in the DC area, it's still not likely to be optimum. This mixture is tried and true.

Make a grid out of wood or plastic strips to form 1 foot by 1 foot squares. Where the grid sections cross, attach them with screws or rivets. 

Plant your seeds, spacing them as recommended on the seed package. Feel free to mix things up; put flowers seeds in one square and veggies in another.

Water only as much as is needed. The soil mix holds water very well. The container holds warmth in the soil better than a regular garden. If you watering early in the season and it's cold, water with warm water from a watering can

When you harvest a square, you can add more compost and plant more seeds.

You can learn more at the Square Foot Gardening website. Be sure and click on "Global Gardening" to learn about Mel Bartholomew's program to spread this great way to produce food to poorer countries.