Instructions to build

1. Setup Build Environment
Open the terminal and enter the following :

sudo apt-get install git ccache automake flex lzop bison \
gperf build-essential zip curl zlib1g-dev zlib1g-dev:i386 \
g++-multilib python-networkx libxml2-utils bzip2 libbz2-dev \
libbz2-1.0 libghc-bzlib-dev squashfs-tools pngcrush \
schedtool dpkg-dev liblz4-tool make optipng maven libssl-dev \
pwgen libswitch-perl policycoreutils minicom libxml-sax-base-perl \
libxml-simple-perl bc libc6-dev-i386 lib32ncurses5-dev \
x11proto-core-dev libx11-dev lib32z-dev libgl1-mesa-dev xsltproc unzip

2. Download device source:

git clone --depth=1 https://github.com/draand28/Xiaomi_Kernel_OpenSource.git

3. Download a compatible GCC toolchain: 

git clone https://android.googlesource.com/platform/prebuilts/gcc/linux-x86/aarch64/aarch64-linux-android-4.9 toolchain

4. Download the Qcom CLANG : https://developer.qualcomm.com/download/sdllvm/snapdragon-llvm-compiler-android-linux64-609.tar.gz

5. Extract the Qcom CLANG using: tar vxzf snapdragon-llvm-6.0.9-linux64.tar.gz

6. Download the device-tree-compiler attached below and place it in /usr/bin    https://forum.xda-developers.com/attachment.php?attachmentid=4829019&d=1569559578

7. Build the kernel with the following:

rm -rf out
mkdir out
export ARCH=arm64
export SUBARCH=arm64
export DTC_EXT=dtc
export CROSS_COMPILE=${PWD}/toolchain/bin/aarch64-linux-android-
make O=out REAL_CC=${PWD}/toolchains/llvm-Snapdragon_LLVM_for_Android_6.0/prebuilt/linux-x86_64/bin/clang CLANG_TRIPLE=aarch64-linux-gnu- andromeda_user_defconfig
make -j$(nproc) O=out REAL_CC=${PWD}/toolchains/llvm-Snapdragon_LLVM_for_Android_6.0/prebuilt/linux-x86_64/bin/clang CLANG_TRIPLE=aarch64-linux-gnu- 2>&1 | tee kernel.log

8. Browse to /out/arch/arm64/boot & you'll find the Image.gz-dtb file (compiled zImage)
   Download the anykernel template and add your Image.gz-dtb file to the archive.
   Boot in TWRP, backup your stock kernel & flash the anykernel zip.