Once the lesson starts there are a few separate ways to mark attendance. The easiest way is through the lesson room itself but there are some other options too.
During the lesson – in the lesson room
When the lesson starts you can click “Lesson room” on the left-hand side:

Within the lesson room click the tick icon on the left-hand side:

Within the pop up you can mark the student as present or absent and select the attendance code, if these have been set up by your admins:

If you’re not currently using attendance codes this will instead ask you to select who is chargeable for this lesson: the school; the learner; or a free lesson.
From the attendance area
Click the Attendance option on the left-hand side:

This brings up a calendar view of all your lessons. Click the “?” on the relevant lesson and then click edit:

This opens the same dialog box as the lesson room where you can mark a student as present or absent and select the attendance code.
From the learners area
On the left-hand side go into your learners:
Select the relevant learner and click on the lessons tab at the top
This brings up a list of all that learner’s lessons. Find the relevant one and click edit to change their attendance.