Open your composition in After Effects
Move the Current Time Indicator to the frame you want to mark
Press `*` on the numeric keypad to add a marker
Or right-click the timeline ruler and choose `Add Marker`
To mark a layer, select the layer first and press `*` on the numeric keypad
Double-click a marker to rename it or add comments
Drag markers in the timeline to reposition them
Use composition markers for timeline-wide notes
Use layer markers for notes tied to a specific layer
Use the Marker panel to edit marker settings if needed