Your first step is to download, compile, and install these two packages. They may be retrieved from their respective homepages: ucspi and daemontools. They should compile and install fairly easily.
The man pages for each of these programs are clear and simple, and I strongly urge you to read all of them. You now have all the pieces of the puzzle. This document will try to help you see the big picture and put them all together.
tcpserver -u 7770 -g 2108 0 smtp /var/qmail/bin/qmail-smtpd &
Replace 7770 with your qmaild uid, and replace 2108 with your nofiles gid. Don't forget the &.
By default, tcpserver allows at most 40 simultaneous qmail-smtpd processes. To raise this limit to 400, use tcpserver -c 400. To keep track of who's connecting and for how long, run (on two lines)
tcpserver -v -u 7770 -g 2108 0 smtp /var/qmail/bin/qmail-smtpd \ 2>&1 | /var/qmail/bin/splogger smtpd 3 &