From the right side options menu on the Sources Tool main page, select the "Evaluate this Source" option.
This takes you to a page where you can add or update your evaluation of this source. You can evaluate sources on several criteria: relevance, evidence, author, publisher, comprehensiveness and timeliness. To learn more about the suggested criteria, open the advice windows by clicking on the provided links.
When you are finished entering your evaluation, save it by clicking on the "Save Evaluation" button at the bottom of the page.