Hey there,
It depends on what kind of system you're using, but the short answer would be to add a line in most Unix to /etc/inet/inetd.conf or, in Linux add an entry in /etc/xinetd.d for it.
For both you can just borrow from what's probably already there.
In unix, in /etc/inet/inetd.conf, copy any line (probably best at the top) that has the terms "tcp" and "stream" in it and substitute the path to your telnetd and telnetd/telnet as necessary - then kill -1 `ps -ef|grep "[i]netd"|awk '{print $2}'`
For linux, cd into /etc/xinetd.d and copy a file, like maybe vsftpd and modify also - rename the file to telnet. Then just run /etc/init.d/xinetd restart.
Hope that was of some help :)
, Mike