I would guess the definitive source is Neal's most recent specification, specified in Dropbox. Of course, new packagers starting out might not have access to that, but once you've learnt how to do it and been accepted it shouldn't be too difficult to adapt to the specific requirements of pclos.
The first stage is to learn the basics, which will be similar for all distributions that use RPM, so read the online guides to learn how to do it. RPM is very fully documented by its authors (originally Red Hat and still closely connected with them) and there are also some useful insights on Mandriva pages (although they use a fork of RPM called RPM 5). Once you can produce working packages, then is the time to consider the pclos way by reading the wiki and asking questions in the forum.
There are still different opinions and you only have to pick two random packages and put their spec files alongside each other to see that. As long as what you do is acceptable to Neal and doesn't break things, it should be OK.