0. Where to buy: Belts can be often bought on Ebay. They are either advertised specifically for the Amstrad CPC but also those for Spectrum+3 will work. The correct belt is 3mm wide, 0.6mm thick and has a total length (outer circumference) of 230mm (often also advertised as 115mm). If there is no other source available, one could look on AliExpress for belt sets (3mm wide, 0.6mm thick) which contain between 50 and 100 belts of different sizes. Usually 2-4 will have the right diameter, but it's a bit of a gamble.
1. Remove the screws in the case of the CPC. You have to remove all screws on the bottom on the CPC and both screws on the side of the CPC. Now you can turn down the upper part of the case (Warning: please remember that the keyboard is connected to the board of the CPC).