This is a template derived from a script I wrote with some ideas taken from https://gist.github.com/pklaus/931579 with the original scripts coming from Ricardo Gameiro, Philipp Klaus and Daniel Jenkins.

mkfirstboot.sh

#!/bin/bash
#Template for first boot script creator.
#feel free to modify this to make use of arguments, allowing for script installation on a different volume.
#I have made use of nowdoc syntax to intepret the enclosed strings as literal strings with no parsing done.
#Harrison Mclean a.k.a h4rm0n1c


#better than using cd, we pop this directory off of the stack at the end of the script.
pushd /System/Library/StartupItems
mkdir FirstRun
chown -R root:wheel FirstRun
chmod -R u+rwX,g+rX,o+rX FirstRun

cat << 'EOF' > FirstRun/FirstRun
#!/bin/bash

#Begin Post-install (first boot) script.

#Insert what you want to run once on first boot here, I've used this for dock plists, default preferences, just about anything really.

#End Post-install (first boot) script.

rm -rf /System/Library/StartupItems/FirstRun/
EOF
chmod u+x,g+x,o+x FirstRun/FirstRun

cat << 'EOF' > FirstRun/StartupParameters.plist
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE plist SYSTEM "file://localhost/System/Library/DTDs/PropertyList.dtd">
<plist version="0.9">
    <dict>
        <key>Description</key>
        <string>FirstRun Script</string>
        <key>OrderPreference</key>
        <string>Late</string>
        <key>Provides</key>
        <array>
            <string>FirstRun</string>
        </array>
        <key>Uses</key>
        <array>
            <string>Disks</string>
        </array>
    </dict>
</plist>
EOF

#back where we started
popd

Note: This script uses Apple's older Startup Items system to execute, which is officially Deprecated.
Rewriting this to use the launchd system would be a good idea.