The kernel of NetBSD is roughly 150,000 lines and can be a bit daunting to tackle. Most people who first try to hack the kernel get what we call kompernelphobia, which is the feeling of being overwhelmed by the size and complexity of something. Drawing a map of the kernel or writing down the calling sequence can help in seeing an overall picture of what the kernel is doing. This is especially useful in the assembly part of the kernel, where, for example, it may not be obvious how memory is being initialized. When reading kernel code, it is helpful to have the editor be able to jump to the definition of a identifier. The program ctags can be used to build a file that the editor can use to follow identifiers.
There are currently around a dozen people actively involved in porting NetBSD to various machines, and it is important to use them as a resource when you get stuck. This is both because they may have an answer to your question and because you may be approaching the problem the wrong way. The four core members of NetBSD can be reached through e-mail at core@NetBSD.ORG. The members of the team that ported NetBSD to the Macintosh can be reached at alice@acm.vt.edu.