Very easy. An MP3 CD is basically just a data disc, so think of it like your HDD - data storage with a bunch of data files on it. Pop the CD into your PC, open it in folder view so you can see the folders/files on there, and copy the contents (all off it if you want everything on there) into a folder on your computer. For the sake of ease we shall say you created a folder on the desktop called "MP3s".

Then all you need is a burning programme. There's loads out there, but I like Nero. You can use whichever you want of course, Windows even has it's own inbuilt programme for it (usually shows up in the purple column to the left in a folder)

Anyway, open up the burning app, Nero or the like, and click on MAKE AUDIO CD. Select the tracks you want by navigating to the folder we created on your desktop (or wherever you put the mp3's). Double check they are in the right track order. If so, hit BURN or follow whatever prompts (a little tip before you start burning the CD is to see if the programme has the option to add or not add gaps between tracks. In Nero it has a check box on the window where you add the songs you can tick so it DOESN'T put in gaps between songs, so you need to see if whatever program you are using is set to ADD or NOT by default)

Then sit back - how quickly the disc is created depends on the discs you used and the performance of your PC. I find two things here worth noting are 1) I tend not to use the PC when it's burning - a) it's quicker and b) you usually get a better burn, and 2) burn at a low speed. People who burn at 52x usually get shit burns, burn at 4x might be slow, but you get a better copy