Read it first : https://github.com/android-rpi/local_manifests

# Build Kernel
 $ sudo apt install gcc-arm-linux-gnueabihf libssl-dev
 $ cd kernel/arpi
 $ ARCH=arm scripts/kconfig/merge_config.sh arch/arm/configs/bcm2711_defconfig kernel/configs/android-base.config kernel/configs/android-recommended.config
 $ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make zImage
 $ ARCH=arm CROSS_COMPILE=arm-linux-gnueabihf- make dtbs

# Install python mako module
 $ sudo apt install python-mako

# Build Android source
 Continue build referring to http://source.android.com/source/building.html
 $ source build/envsetup.sh
 $ lunch rpi4-eng
 $ make ramdisk systemimage vendorimage

 Use -j[n] option with make, if build host has a good number of CPU cores.

# Prepare sd card
 Partitions of the card should be set-up like followings.
  p1 128MB for boot : Do fdisk, set W95 FAT32(LBA) & Bootable type, mkfs.vfat
  p2 768MB for /system : Do fdisk, new primary partition
  p3 128MB for /vendor : Do fdisk, new primary partition
  p4 remainings for /data : Do fdisk, mkfs.ext4
 Set volume label of /data partition as userdata
  : use -L option for mkfs.ext4, and -n option for mkfs.vfat
 
# Write system & vendor partition
  $ cd out/target/product/rpi4
  $ sudo dd if=system.img of=/dev/<p2> bs=1M
  $ sudo dd if=vendor.img of=/dev/<p3> bs=1M
  
# Copy kernel & ramdisk to BOOT partition
  device/arpi/rpi4/boot/* to p1:/
  kernel/arpi/arch/arm/boot/zImage to p1:/
  kernel/arpi/arch/arm/boot/dts/bcm2711-rpi-4-b.dtb to p1:/
  kernel/arpi/arch/arm/boot/dts/overlays/vc4-kms-v3d-pi4.dtbo to p1:/overlays/vc4-kms-v3d-pi4.dtbo
  out/target/product/rpi4/ramdisk.img to p1:/