When you live in a northern climate as we do, one of the most important things you can learn about gardening is how to extend your season. It's not enough to get started at the last average frost date. To grow a wide varity of plants, you must learn how to get the season started early and keep it going late despite the weather.
Raised bed gardening is becoming popular in many different climates because of it's many advantages. Beds can be built at a comfortable height for any gardener even those in wheelchairs. Beds also have a very neat and tidy apperance. In short season climates they can have the distinct advantage of warming early. Because they sit above the ground level and are surrounded on four sides by the air, the soil in a raised bed will warm up quicker then the soil of a regular ground level garden.
In very early spring you can speed up the warming of your garden soil by covering it with clear plastic. Clear plastic has been shown to do this much better then black plastic so if you really want the benefit of this process, make sure you use clear plastic. If you have raised beds and they are not too large you can also use window panes for the same effect. This solarizing power is so effective, if your outside temperature is warm, it can be an effective weed killing tool.
The cold frame is really just a low tech greenhouse and is a great way to protect those early spring and late fall crops. The walls can be made of many different materials. They may be solid and permanent, as with a wood frame nailed together, or more temporary such as bales of hay. The top should be a clear plastic or glass covering.
The frame may be built to any desired dimensions to cover a particular crop, but especially with more permanent frames, such as a wooden box, may best be kept small enough to be portable. That way you can move it around for use on different areas of the garden as needed.
Make the front wall of the frame lower then the back wall. The farther north you live, the lower the sun hangs in the southern sky in the early and late season. Therefore, if you have a tilt to the cover of your box, you will catch more of those warming rays that you are aiming for.
Lastly make sure you don't permanently attach your cover to whatever you choose for your walls. That way when the days are heating up, but the nights are still cool, you can partially or even completely uncover the box during the day to prevent it from getting too hot in there.