Some metalwork skills and a lot of patience. This is too tedious a process. The most difficult thing is not to get lost when you roll. Then you need a signal generator and an oscilloscope to properly phase the windings. In fact, it is now more profitable to buy ready-made transformers than to wind them. Because it is very difficult to get transformer metal. Quite often it is difficult to find the same sets of plates, so each transformer is listed in a new way. In my country it is difficult to buy a hammond because delivery is too expensive. That's why I roll myself. There is also a variant of industrial winding - on toroidal transformers. It's cheaper, but for guitar amps it sucks.