Setting Up Your Sketch
For this painted study, you can scan a traditional sketch and open it (as I did), or draw a sketch in Photoshop.
In this project, I cut my scanned drawing to a layer by leaving the Background layer white, choosing Select > All, and then choosing New > Layer Via Cut from the Layer menu. Next, set the blend mode of your "sketch" layer to Multiply in the Layers panel (see Figure 5), so the white areas of the sketch will appear to be clear, which will allow you to use your sketch as a guide later, when you paint color on underlying layers.
The sketch layer is set to Multiply in the Layers panel.