This is done millions of times a week, I am sure. I just want to write it down so I hope to imprint it on my brain. It is very simple with Gentoo.
rc-update is the tool that Gentoo uses to abstract the guts that often are associated with adding a service to certain runlevels.
Example with adding ntpd to the default run level.
rc-update add ntpd defaultNAME
RC-UPDATE MAN page details.
rc-update - add and remove init scripts to a runlevel
rc-update add script <runlevels>
rc-update del script [runlevels]
rc-update show [--verbose] [runlevels]
Gentoo's init system uses named runlevels. Rather than editing some
obscure file or managing a directory of symlinks, rc-update exists to
quickly add or delete init scripts from different runlevels.
All scripts specified with this utility must reside in the /etc/init.d
directory. They must also conform to the Gentoo runscript standard.