A cold frame, as defined by the University of Missouri Extension, is a protected plant bed. It is a rectangular box with a back higher than the front and a slanted lid covering the top. The inside of a cold bed is anywhere from five to ten degrees higher than the outside, which improves the chances of certain plants surviving a cold spring night. A cold frame is also used to "harden" of seedlings that were started indoors by slowly introducing them to a colder weather without shocking their system.
Measure out an area of 3 feet by 6 feet for the cold bed. This is the average size of a small garden, says the University of Missouri Extension.
Dig out the measured area to a depth of 14 inches.
Place 2 by 4 inch by 2 foot boards in the back corners of the excavated area, pounding them a few inches into the dirt.
Place two 1 by 10 inch by 6 foot boards at the back of the cold bed and nail them to the 2 by 4 pieces set in the back corners.
Place one 2 by 4 inch by 2 foot piece of lumber in each of the front corners of the excavated area and pound into the dirt.
Place a 1 by 10 inch by 3 foot piece of lumber on each side of the garden and nail it to the anchor pieces in the four corners.
Cut two triangular pieces of wood to finish off the sides of the cold frame. The pieces are 1 by 10 in size.
Place a 3 foot by 6 foot glass sash on top of the cold frame. Glass sash pieces are available at most construction and gardening centers.
Add soil and compost to the inside of the cold bed area and plant your garden.