Do you have any readily available scripts that I can use?
As a standard service, you can simply execute:
$SPLUNK_HOME/bin/splunk enable boot-start
as the root user and Splunk will generate an appropriate script into /etc/init.d/ and link it to all the right /etc/rc locations. You can add a -user username option to generate the script to run as a non-root user.
Do you intend to have Splunk managed as an SMF (with a manifest and so on) or as a legacy service (with a startup script in /etc/init.d)?