Create a directory for the source files & Go into created directory

mkdir matrixx
cd matrixx

Initializing Repo

repo init -u https://github.com/Matrixx-Devices/android.git -b 15.0 --git-lfs

Now sync sources

repo sync -c --no-clone-bundle --optimized-fetch --prune --force-sync -j$(nproc --all)

Now run

. build/envsetup.sh
brunch devicecodename

Build Flags

# To include Gapps 
WITH_GMS := true

# To Build Pixel launcher
TARGET_INCLUDE_NEXUS := true

#Some more GMS Flag
TARGET_SUPPORTS_GOOGLE_RECORDER := true
TARGET_INCLUDE_STOCK_ARCORE := true
TARGET_INCLUDE_STOCK_AICORE := true

To Add Maintainers name

# Add in your overlay (overlay/packages/apps/Settings/res/values/cr_strings.xml)
<string name="matrixx_maintainer">Diddy</string>