http://www.unofficialbmw.com/e36/drivetrain/e36_final_drive_fluid_change.html
It's not a hard process, and fortunately for you, the gasket is between 3 and five dollars from BMW.
Follow the directions for the fluid change, but when all the fluid is out, unbolt the cover and replace the gasket. Then you bolt it back on.
Few tools you'll need that arent common, but they're available at napa for a pretty reasonable price.
Good luck.