1. Grabbing the source

Repo is a tool provided by Google that simplifies using Git in the context of the Android source.

1.1 Installing Repo

# Make a directory where Repo will be stored and add it to the path
$ mkdir ~/bin
$ PATH=~/bin:$PATH

# Download Repo itself
$ curl https://storage.googleapis.com/git-repo-downloads/repo > ~/bin/repo

# Make Repo executable
$ chmod a+x ~/bin/repo

1.2 Initializing Repo

# Create a directory for the source files
# This can be located anywhere (as long as the fs is case-sensitive)
$ mkdir crDroidMod
$ cd crDroidMod

# Install Repo in the created directory
$ repo init -u git://github.com/crDroidMod/android.git -b 9.0

This is what you will run each time you want to pull in upstream changes. Keep in mind that on your first run, it is expected to take a while as it will download all the required Android source files and their change histories.

# Let Repo take care of all the hard work
$ repo sync --force-broken --force-sync --jobs 16 --no-clone-bundle --no-tags

# Go to the root of the source tree...
$ cd crDroidMod
# ...and run to prepare our devices list
$ . build/envsetup.sh
# ... now run
$ brunch "your device name"