Go to "Sessions" tab to see all existing sessions to start taking attendance.

Click "Take attendance"  button by clicking the triangle icon.

Change status to "All" if you wish to choose the same for all students.

Click the radio button to mark all students.

If you wish to mark each student individually, then set status to "unselected" and tick respective radio buttons next to each student name.

Once all students are marked, click "Save attendance".

Click "Report" tab to see all students attendance in all sessions.

You'll see which sessions students were there based on the categories given and grades they receive.

How to Take Attendance on Moodle