Creating Wave Line Art Style Background

Start by applying a blue to black gradient in diamond mode.

Create a rectangle using rectangular shape tool.

From filter menu select "Distort>>Wave". Apply settings as shown.

Duplicate the layer several times and place below each other.

Select and merge all the wave layers. Double click the layer to open layer style window. Apply settings as shown.

Change the layer mode to "Luminosity" and opacity to 50% from the layer palette.

Duplicate the layer. Duplicate the layer. Press Ctrl+T. Right click and select rotate 90°.

Create a new layer. Select brush tool and press F5 to open brush tool.

Select leaf brush from the custom brush and apply as shown.

Create a new layer. Select star brush from the custom brush and apply as shown.

Apply a blue to black gradient in radial mode.

From filter menu select "Sketch>>Halftone pattern". Apply settings as shown.

Change the layer mode to "Color Burn" and opacity to 20% from the layer palette.

Select and merge all the layers. Select eraser tool. Right click on the screen and select the brush as shown in the image below.

Apply eraser tool on the edges of the document as shown.

Add some custom shape. Play around with their layer modes.