First you need to get your hands on a Dreamcast dev console, since DC discs are burnt and read from outside to inside, backwards from what normal discs do.
Then you need to get your hands on the compatibility disc will_chellam mentioned.
After that, you need to develop a program that will invert every bit of the disc, if it's a 1, it'll become a 0, and vice versa.
After that, re-compile, and burn it using the dreamcast dev console.
Once that's done, print off a label, and using glue, stick it to the data side of the disc so that the adhesive of the label faces outwards.
There you have it, a backwards compatibility disc.