On the Discussions List page, from the context menu of the topic you want to evaluate, click Assess Topic.
Click the Topic Score link beneath the name of the user you want to assess. This will bring up all of their posts for the topic, which you can then assign either individual scores or an overall topic score for.
To assess individual posts, in the Assessment tab of the Edit Topic page, select the Allow assessment of individual posts check box. Clear the check box to assess the topic as a whole.