Why Feedback is Important:
Feedback is important because it helps us find out how a member is. In a site like torrentinvites its one of the most important parameter with so many scammers around.
How to Add Feedback:
Follow the below steps to Add feedback (all required field explained) -
1. Go to the users profile you want to leave feedback for & left click on his username (at any thread/ post), it will pop-up a menu (shown below).

2. Once there click on the feedback tab.

3. You should see 2 options as below.a. Click on Submit Feedback For Member
b. View Complete Feedbacks For Member
a. Click on Submit Feedback For User
4. After you click on Submit Feedback For User you'll be redirected to a new page.

Now fill in the details:
You were the: You role in sell/ buy or trade

1. Seller (Seller receives a feedback by the buyer once the deal is completed)
When you should choose it - At
TI only TheTrader is allowed to sell, therefore only he can use it.
2. Buyer (Buyers receives a feedback by the seller once the deal is completed)
When you should choose it - When you receives the invite, account or bonus in any GiveAway then you will choose this option.
3. Trade (If it was a trade both the people receive feedbacks)
When you should choose it - Both member who involve in Trade of invite/ account will choose this option.
Overall experience: Your experience with that user in that particular trade.

1. Positive (+1) [Buy/ Sell / Trade is sucessful]
2. Neutral (-0-) [No benefit of adding it]
3. Negative (-1) [Something went wrong]
Other fields:

3. A short comment: (Visible to all) A short comment about your experience.
4. Additional comments: These comments are visible to the user who receives feedback and the staff.
5. Once all above done click on 'Submit' at the bottom of the page.
b. View Complete Feedbacks For Member

Important Points to Remember While Adding Feedback -
1. Never try to cheat 'Feedback' system, adding/ asking for false feedback can ban you from TI
2. 'Feedback' should be left after buy/ sell/ trade is successful. Asking for it before is not allowed (feel free to report any such member to staff)
3. Try to add 'Feedback' after few hours/ days on receiving invite/ account specially for high level trackers. It will make tracker staffs hard to trace it. [Not a rule, Just a suggestion]
4. It will better not to add tracker name in 'A short comment' as it visible to all, you are free to add anything in 'Additional comments' as it visible only to that member or TI staff. [Not a rule, Just a suggestion]
That's it. You just left your first feedback. I hope this tutorial was useful.
Good luck! 
Feel free to ask/ suggest if something is missing/ not clear.
Thanks to @TheTrader @Insideman @slash @tjay for support.