Drawing on layers
The layers of a canvas are listed in the canvases sidebar. To open it, select Canvases from the View menu or click the Canvases toolbar button.
Click the disclosure triangle next to a canvas to see its layers. Layers are transparent, so behind the top layer's contents you see the second layer's contents, and so on to the contents of the bottom layer.
Each layer in the list has a small preview of its contents, a title, and when you put the mouse pointer over it, three icons. The layer you are currently editing has a pencil icon 
to its left. To edit a different layer, click just to the left of its preview and the pencil icon moves there.
to its left. To edit a different layer, click just to the left of its preview and the pencil icon moves there. To make a layer visible or invisible, click its eye icon 
. To set whether a layer appears when the canvas is printed, click its printer icon 
. To lock or unlock a layer, click its padlock icon 
. Locking a layer prevents its contents from being changed.
. To set whether a layer appears when the canvas is printed, click its printer icon . To lock or unlock a layer, click its padlock icon . Locking a layer prevents its contents from being changed. You can rearrange layers by dragging them in the list. You can also drag a layer to move a it to a different canvas; Option-drag to copy it instead. If you drag it to a completely different document, it will always be copied there.
To add a new layer click the new layer button at the bottom of the canvases drawer. 
To delete a layer, select it and then press the Delete key or choose Delete Layer from the Action pop-up menu at the bottom of the sidebar. Any canvas needs at least one layer, so you can't delete the last one.