How to port forward to play on Garena?

Hello, me and my friends are trying to use Garena client to play warcraft 3 online throught Belkin f5d7234-4 v1 router. We can see most games and join them however almost 90% of the time either me or him will get kicked out once the game starts.If either of us hosts everyone else will lag and eventually quit. I heard forwarding ports will fix the issue, i tried but have not succeeded. Can anyone explain how to do it.Thanks in advance!