Technically skilled gardeners can build their cold frame by their own. For doing that, just timber a wooden frame and a wooden roof structure and string it with a robust plastic foil or a fleece.
Make sure that ventilation is good. Fix the roof with hinges to the frame and mount a stem – lift the roof, set the stem inbetween – 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 already 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 wind-sheltered. Ideally, the higher backside should point north and the beveled 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 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 when the days get warmer in mid-March.
The best vegetables for sowing in March are: cabbage, garden cress, lettuce, radish and lesser celandine. After mid-April you can also sow cucumbers, tomatoes, zucchini and peppers.