Resources.
Printable templates you can download, print, and fill in by hand. Plus a curated directory of the publishing administrators and collection societies every working songwriter should know about.
Still the best.
what, and why.
Which of these do I actually need?
If you're a songwriter — not a performing artist — the short answer is: one PRO (ASCAP, BMI, PRS, SOCAN, APRA, etc. — whichever covers your country), and one publishing administrator (Songtrust, Sentric, CD Baby, etc.). That gets you the majority of the money your songs earn globally.
If you're also the performer on the recording, add PPL (UK) or SoundExchange (US) to collect your neighbouring-rights income, which is a separate pot of money with your name on it.
If you self-publish in the US, register with The MLC so you get the mechanical royalties from US streams. Everyone else's admin covers this automatically.
A note on choosing between them
Most of these services won't move the needle massively for a writer earning under a few hundred pounds a year. But they will stop you missing out on money that's already been collected with your name on it — money that gets written off as "unmatched" if nobody claims it. The difference between signing up and not is the difference between "I don't earn much from songwriting" and "I earn the same amount, but actually receive it."