In the left sidebar, go to the Games section and click Examining. In the top menu, above the game board, click the Examining Mode button.
In the right sidebar you now have access to arrows, circles, rectangles, ghost pieces and throbbing pieces. You also have a button to undo the last lecture mark or undo all lecture marks.
To draw an arrow, hold down the command key and left-click and drag your mouse from the starting square to the destination square.
To draw a circle or rectangle, select which lecture mark you want in the right sidebar and hold down the shift key and left-click on a square.
To draw a ghost piece or throbbing piece, select which lecture mark you want in the right sidebar and hold down the shift key and left-click on a piece on the board.
To undo lecture marks one by one, left-click on the Undo last lecture mark button to undo the lecture marks one by one. To undo all lecture marks, left-click on the undo all lecture marks button once.