Flash custom built kernel

Need some help/documentation regarding how to flash custom built kernel. I have some experience in the Linux world, but a newbie to android
2 questions here based on the documentation here Fairphone 4 Kernel Source Code — FAIRPHONE open source documentation

  1. I was able to unpack an already build boot.img but when i tried to repack the same set of files, my boot.img is much smaller (36M in place of 96M) and I am not able to flash it to a fairphone either
  2. downloaded the kernel source and built it and also have the build/arch/arm64/boot/Image file and don’t know how to proceed after that

Any help or reference please