There is a sticky that shows exactly how to use iTrader.
Basically, the steps are:
1. Go to the thread you want to give itrader for and copy the URL of the post
2. Click on the iTrader number on the left side of the screen Example iTrader: (0)
3. Click on "Submit Feedback For grafxextreme" or the username you want to leave a rating for
4. On the next page, "You were the:" Select from the dropdown menu whether you were the seller, buyer, trade
5. For the "Overall experience:", select from the dropdown menu whether you had a positive, negative, or neutral experience
6. In the "A short comment (visible to all), type a short description
7. In the "Deal thread URL" text box, paste the URL you copied earlier from step 1
8. In the "Additional comments" text box, write anything you want. This is only visible to buyer, seller and staff only. It also has to be at least ten characters.
9.Then click on the "Submit Reply" button on the bottom of the form.
Your done!
If you need a visual tutorial, check the stickied post. It is an excellent reference.