You should know how to use iptables before proceeding. Having said that...
Here is what I have in my router to forward ftp
iptables -A FORWARD -p tcp -d 188.8.131.52 --dport 21 -m state --state NEW,ESTABLISHED -j ACCEPT
iptables -t nat -A PREROUTING -i eth0 -p tcp --dport 21 -j DNAT --to-destination 184.108.40.206
That may not be the best way to do it, but those would be the lines I would use in MY setup. Yours may be slightly diiferent keeping these from working.