When you have decided the place, position the box, and dig out holes for the corner post. These posts can simply be placed on top of the soil or they can be rooted in the ground several inches or more. This is up to your preference. Just plan ahead. Use 4x4 pieces of lumber.  These will be used for the corners of the boxes.  Cut them to be at least the same height or several inches deeper than the box. If your boxes are going to be longer than 8', you will want add to center posts on the long sides. Using 2x4s, cut boards for the two long sides, equal to the distance from the far edge of one corner post to the far edge of the other. Cut the short end boards to be equal to the distance between the far edges of the posts, plus the ends of the long side boards. Use outdoor appropriate screws; 1"-1 1/2" or decking screws work well. Drill through the boards directly into the posts. Once the box is set, line the box with 1/2" hardware cloth to keep out pests and varmint. Staple or screw the hardware cloth to the sides. Next, place some weed cloth on top of the hardware cloth.  Staple that to the sides of the box. This is to keep the weeds from growing from below. Bring in some planting mix for soil. It is usually measured in cubic feet. Use a wheelbarrow or park close to the box and start filling it up. Stand on it to pack it down. Leave about 2 inches (5.1 cm) from the top. Make sure to fertilize the soil or rotate types of plants in the boxes to keep the soil fresh. Plan to grow plants that suit the season of the year as well.

Summary:
Find the location where you plan to put the box. Prepare your wood posts. Cut the side walls. Drill the boards into place. Place a barrier cloth. Place weed cloth. Add in your soil. Have fun watching your garden grow!