The official document is good at building kernels. There might be some easier install methods “sometimes” (often?) other than flashing. Most people though run into trouble with configuration. If you don’t understand configuration (which is actually somewhat simple, but not always obvious), then you will likely fail. You might find this useful:
- https://forums.developer.nvidia.com/t/topic/262647/12
- https://forums.developer.nvidia.com/t/how-to-fix-xorg-conf-in-jetson-orin/229155/12
Some more on kernels:
https://forums.developer.nvidia.com/t/topic/238718/25
Keep in mind that you should use the exact kernel source that works with the currently running system. This is part of what @WayneWWW is getting at mentioning the Jetson archive. Each L4T release (which is just Ubuntu plus NVIDIA drivers; check “head -n 1 /etc/nv_tegra_release”) is tied to a specific kernel, and that kernel and documentation is available for each release found here:
https://developer.nvidia.com/linux-tegra