Download the AOSP 6.0 sources and sync:
repo init -u https://android.googlesource.com/platform/manifest -b android-6.0.0_r1
repo sync
Then add the manifest and sync again:
mkdir .repo/local_manifests/
curl https://raw.githubusercontent.com/marshmallow-og/manifest/master/geehrc.xml > .repo/local_manifests/geehrc.xml
repo sync
Apply a patch required:
cd frameworks/base
git fetch https://gerrit.omnirom.org/android_frameworks_base refs/changes/71/14171/1 && git cherry-pick FETCH_HEAD
cd ../..
Start build:
source build/envsetup.sh && lunch aosp_geehrc-userdebug
make otapackage