First, check whether the scripts already exist. Normally they are located at /etc/init.d. Some distros use a central configuration file for activating services at boot time.
If not, create new scripts, best one script per service. Use the scripts of your distro, copy and modify them. They aren't just start scripts, but also stop or display the status, you use them by entering: /etc/init.d/example start
Now look for the subdirectories. Search for a script which is started on entring runlevel 3 and stopped on rl 1. Look for the according scripts in rc3.d and rc1.d.
You ought to find a copy of the script in rc3.d and in rc1.d or a symbolic link. These have the name S??example and K??example. S... is used for starting when entering the runlevel and K... for stopping. Check this and ask if anything stays unclear, as your computer might hang on boot or shutdown when you make a mistake.
Enter the links for your script, as said, if they aren't already there.