The first step is to acquire a shaving brush in need of restoration. The best source to get those at the moment is eBay. If you can get a lot of 4-6 brushes, it's a nice little project for a beginner that requires a minimum of tools. Please note that you can restore a brush without power tools.
Getting the brush ready
The old knot will need to be removed in order to measure the size of the new knot. The measurement (diameter) will allow you to order a new knot of your choice to place in the brush. Keep in mind that you must order a knot that is as close as possible to the size of the brush. If the brush is 21mm, look if you can enlarge the hole to 22mm to get a 22m knot. If not, order a 20mm. If the knot is 20mm, do not order a 18mm knot, it will be too small.