Any guide to adding a newer kernel to my custom compile?

If you have to ask, you probably can't.

The kernel is very tightly integrated into OpenWrt, as many of the targets and devices need heaps of non-mainline patches to get them working (and to get the kernel smaller/ using aligned access for mips/ arm, …), so there are multiple layers of different patch series (generic, target, sub-target, backport/ pending/ hack, …) to take care of (to rebase) and corresponding DTS/ setup changes. To cut it short, it's difficult and a manual process, you start with:

  • copying the current patch series verbatim
  • start by rebasing the generic patch layers
    ideally on a platform that doesn't needs any non-mainline patches to work, e.g. x86_64
  • once that part is working, you can start looking into doing the same for your actually preferred target.

don't expect that to be a walk in the park, it isn't.