Cathrin is lacking documentation. Essentially, I haven’t found the time to write it yet. Probably not helped by the fact that I don’t tend to make many corsets.
Beginner corset makers should read the tutorials written by Sidney Eileen, who demonstrates several different construction methods. Foundations Revealed is a paywall site for corset makers,with several exceptional articles free for non members.
Corsets must be made from fabric with no stretch at all. The fabric designed for corset making is called coutil. Although coutil is expensive, not much is needed for a corset. Some cheaper alternatives include canvas, denim and cotton ticking. However, these will stretch over time and are not suitable for any garment with a large waist reduction: they are not strong enough. They are good options for testing the pattern to see how it fits on you. For corsets made from silk and cotton and other fancy fabrics, you need to use several layers - the fabric you want to be seen, on top of a layer of coutil or other strength layer, and perhaps also a third layer made out of lining which will sit next to the body.
Corsets are easier than they seem to construct, however they need several specialist items and tools.
You will need BONING. Historic corsets used whale bone to stiffen corsets. Modern corset makers use steel. You can get steel bones either as flat steel and spiral steel - do research, as different types of corset and different corset makers have their own preferences. Plastic bones are also a cheaper option.
You can find a good guide to other tools at Foundations Revealed.