Hi, Ive never changed them on a C4 but I have done a ton of brakes over the past 35 (yikes) years. First you will need some special tools, a caliper expander for one and what ever kind of fastener removal tool that attaches the calipers.
Then you should remove the rotors, have them checked with a micrometer to see if they are in spec tolerance or warped and then have them turned. You may get some folks that tell you it's ok not to turn the rotors, however if you don't you will develop a problem, if not immediately, but pretty soon afterward.
When you expand the calipers upon reinstallation you need to remove or loosen the master cylinder cover and kep a cloth over it so the fluid does not spray on anything. Brake fluid will remove paint in about 3 seconds. You should also get a bleeding tool and refill/bleeder set up so you can properly refill the system You don't have to do this with discs but you really should. Install the new pads, reinstall the calipers and close the cap and you should be ready to go.
As you can tell this is not a complete and comprehensive instruction, but gives you an overview of a relatively simple process that can become a problem if you are not experienced. I suggest you team up with someone that has done it before so you can learn it properly. If I can be of further help let me know.
markone