The Multiply mode is quite probably one of the most frequently used blending modes. It blends layers to create a darker color, except where there are white pixels. The white pixels will disappear.
When you have found a blending mode that creates the effect you are looking for, select the Stairs palette. Click on the "Create a New Layer" button at the bottom of the Layers palette.
You now have a new empty layer which appears in the Layers palette just above the Stairs layer, but nothing should have changed on your image in the document window.
When you add a new layer the layer is empty and transparent by default.
We want to use this new layer to draw a border around the image you've been working on. The advantage to drawing on this new layer, as opposed to drawing on any of the other layers, is that it can be isolated on its own – turned on or off, transparency adjusted and other adjustments applied to the layer.
Again, make sure that the new empty layer is selected. Grab the Brush tool from the Toolbox (or hit B on the keyboard). On the tool options bar, click on the Brush Preset Picker and select a rough looking brush. (I picked a Dry Brush Light Flow) and set the master diameter to about 100 pixels. On the toolbox, set the foreground color to black by hitting "D" on the keyboard – this sets the foreground and background colors to their default – black and white. Start drawing around the edge of the image to create a rough border.
Rename the layer as "B order". If you need to remember how to do that read back over step 1.
The main benefit to drawing on this new empty layer is if you don't like your work you can turn it off without having it affect your entire image. You can also play with the opacity to see how that looks.
Now that you have the basics of blending down try to play around with your image and add additional layer to your image, perhaps draw some more. Choose different brushes and colors and change the blending modes.
So that's it for our tutorial on blending modes and opacity. Obviously the best way to find out how they work is to experiment and check out all the different effects you can achieve.