How can I create my own build of LineageOS for an unsupported device?

Question

lineageOS

Let's say there is a smartphone (Umidigi S5 Pro) for which there are not even unofficial assemblies. Are there detailed instructions for building the same LineageOS for an unsupported device?

Those instructions that I found consider the option that the kernel for the device has already been compiled by someone. For my smartphone, I did not find a repository.