Compared to a greenhouse the cold frame is much smaller, but it functions in much the same way by letting light in and protecting the plants from the cold. As it is portable it can be quite easily moved wherever it is needed. Also, it is much cheaper than a greenhouse and therefore more affordable for all gardeners.
Technically skilled gardeners can build their cold frame. For doing that you require a timber wooden frame and a roof structure that can support a robust plastic or glass cover.
Make sure that ventilation is good. Fix the roof with hinges to the frame and mount a stem – lift the roof, set the stem in between and ventilation is done. If you want a more professional cold frame use polycarbonate plates for the roof. This roof is much more solid and insulating. The cold frame will heat up noticeable with the first weak sunrays and it will keep the warm air inside, thus your vegetables will have the right climate to grow faster.
The correct position for a cold frame is an entirely sunny place. Furthermore it should be placed next to the house, so that the cold frame is protected from the wind. Ideally, the higher backside should point north and the bevelled side south. This way, your cold frame catches most of the sunlight.
After you have finished building your cold frame you are ready to sow, but ensure you aerate the soil first and do not forget to water. If it is still very cold outside, you should also protect your plants with a fleece or straw. Start with ventilation as the days get warmer.