Copy the link of the trade thread > go into the profile of the person you traded with (by clicking on the persons username) > click into the feedback section > click leave feedback > paste the thread link and leave your comment.
Nah you have to do that manually. If it was you that started the thread, Open thread > Click Edit Post in the first post > click Go Advanced > then where it say In progress click the drop down menu and select Completed then click save.
