When you're in the process of grading an assessment, there is an option at the bottom, labelled Overall Assessment Feedback. Simply fill out this section and click Save Feedback.
To get help with marking assessments, please see this article.
If a test contains no Short Answer questions, you will not be prompted to grade the assessment. In this scenario, click on the Course that contains the assessment that you wish to leave feedback for.
Next, click on the Users & Groups tab and click Details for the user you wish to add feedback for.
On the next page, find the assessment and click Details beside it. This will expand the assessment and display every attempt of the user. Click on the attempt that you want to leave feedback for.
On the next page, just scroll to the bottom and add your feedback.
When a learner views their assessment attempt, you feedback will be displayed at the bottom.