Man-made nest boxes are useful where there are no natural caves, no longer any old and decayed trees, or no suitable recesses on buildings.
Not only songbirds use nest boxes for brooding or as a warm shelter in winter – sometimes hazel dormice or wood mice and even a fat dormouse will use it during the winter. Therefore you should knock before starting to clean. That warns possible winter guests and they have time to leave their shelter. In addition to that, you should observe the nest box for a few days to make sure there are no early candidates for brooding.
For the actual cleaning remove all old nest material and give the box a dry brushing. If you feel there are too many parasites in it, you can rinse it with water or soda lye. Do not use disinfectants or other chemicals for a birds' nusery.
You can easily make nest boxes yourself. Not only is it fun, it is also an excellent way to get children and teenagers acquainted with animals and the way they live. A full and 'booked-out' nest box can only be expected, however, if your garden is in harmony with nature and the birds can find enough food there.