in any post of the person for whom you wanna give an i trader, you would see a green square on the right.... click on it and follow it... quiet simple actually..
Yeah, click on the number of their itrader from their profile (click on their name). From there, once you can see others who left itrader, there is an option at the top left to "Submit itrader for ______". Just use that.
You need to find his posts in the Buy/Sell/Trade section or visit his profile where you will see his iTraders and where you can add iTrader for him.
Another tip is to be patient before leaving one. Don't rush to hand out negative itraders if the client has gone a few hours without replying