Click "Add an activity or resource" to add assignment activity. It can be hidden, since students don't need to submit anything.

Choose "Assignment" from the activity picker.

Click the "Assignment name" field and add a name for the activity.

In the "Submission types" section, at "Submission types", both the "File submissions" and "Online text" check boxes are de-selected

Under "Feedback types", select the kinds of feedback you want to be able to deliver using the Moodle interface. 

You may want to enable an "Offline grading worksheet" so that you can download and upload a worksheet with student grades when marking the assignments.

If you wish, enable "Comment inline" option.

Click "Save and return to course" button.

Once the assessment is completed, you can leave the feedback for students. Click on the activity name.

Click "Grade" next to student you want to grade and leave feedback.

Since there are no submissions, the only available fields are for grading and feedback.

Click "Save changes" when done with marking.

Create an offline Assignment