Tutorial 8: Layers
Layers contain different sets of objects on the same canvas. We'll put our background image on a new layer to keep it separate from the character diagram.
Click the Canvases button in the toolbar to see the canvases sidebar. It contains the one canvas you have so far; click the disclosure triangle next to it to see its layers.
You only have one layer right now, so we'll add another one. Click the Add Layer button 
below the sidebar. A new layer is born. You can name it if you like; something like "Background" should suffice. 
below the sidebar. A new layer is born. You can name it if you like; something like "Background" should suffice. The order of layers is important; stuff on higher layers appears in front of stuff on lower layers. We want the background to appear behind our relationships diagram, so drag the new layer and drop it below the other one in the list.
Let's concentrate on the background layer. The layer you're working with right now has a pencil icon to the left of its preview in the drawer. If the background layer doesn't have the pencil icon, click just to the left of the layer preview to put it there.
For your background, find a nice image on one of your disks (or download one from the Web). Drag the image file from the Finder and drop it on a blank area of the canvas. A new shape containing the image is created on the background layer. 
Drag the selection handles on the corners of the shape to stretch it to a nice big size; hold Shift as you drag to keep the image from getting distorted. Then use the Image inspector to set the image's opacity to about 15%. 
Now you have an attractive, informative diagram. You know how to use the outline view to create objects, how to label and connect them, and how to add styles. You know how to propagate styles around to many objects. You know how to use multiple layers. At this point, you're pretty much unstoppable!