Make sure you have all the prereqs. If you don't know or haven't installed them. Go here. http://source.android.com/source/initializing.html Also go to the next page and install repo.

Once you are done with that copy sync.sh and build.sh into the directory you want the source code to be.
./sync.sh (then follow the on screen instructions)
./build.sh (then follow the on screen instructions)
And your done!

If you have already run sync.sh once. you dont have to again. Instead before you run ./build.sh Type "repo sync -j10" then run build.sh