Create a Long Shadow Background Using Shape Layers:
Step 1: Set the Foreground color to Black and pick the Rectangle Tool. SHIFT + click and drag to create a rectangle that covers pretty much the whole background. The new shape layer should be placed right on top of the Background layer.
Step 2: Go to Edit > Free Transform (CTRL/CMD + T) to enter the Free Transform Mode. Then, press and hold the SHIFT key and rotate the black rectangle -45° (counterclockwise).
Step 3: Now click and drag the sides of the black rectangle until they reach the corners of the red rectangle.
Step 4: Click and drag the left part of the rectangle until it reaches the center of the red rectangle (diagonally). When you're done, hit ENTER/RETURN to accept the changes.
Step 5: Change the black rectangle layer's Fill value to 0.
Step 6: Double click the black rectangle's layer to apply a Gradient Overlay effect. Start by choosing a Black to Transparent gradient. Then, change the Angle to -45, and the Opacity to a value you like, depending on how subtle or intense you want the shadow to be. The value used here is a quite subtle one, which is 15%.
Step 7: This is the most basic way of creating a long shadow for simple shapes.
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