Hi there!  CT Member Kacy here with a tutorial on using a custom brush for a layout background (with the shape tool).

First, open a new document with a white background.  Next, right-click on the Shape tool and select Custom Shape Tool.

Choose the shape you want to work with, and draw this on the document – Photoshop will automatically make a new layer.  Hold down the Shift key while you drag your mouse to keep the vertical and horizontal proportions even.

Next, use the Marquee tool to select the shape.  Go to Edit, Define Brush Preset.

A dialogue box will pop up – give your new custom brush a name you will remember later.

Now we are going to “paint” on the background.  I always like to do this on a new layer – it’s easy to delete later (if you decide you don’t like how it looks), and you can adjust the size, fill, and opacity of the layer later.

Select your brush tool.  Then, from the drop-down box at the top, scroll to your new custom brush (it’ll be at the end of the list).

Next, open the brush palette (this is usually located in the right third of your screen).  We are going to adjust some options so that the brush varies in size and opacity when you paint, and you don’t have to manually adjust the brush each time you paint on the background.

First, we are going to change the Shape Dynamics (click the check box and select the title) – here, I’ve adjusted the size and angle options to 100%.

If you want to adjust where the shape appears when you click your mouse, adjust the Scattering. I have left all of the scatter options at the defaults for this example.

Next, I have adjusted the Transfer settings to randomly change the opacity of each shape for a little variety.

Reselect the brush tool, and on a new layer (or layers), paint the shape until your background looks the way you want.  Remember, the brush size you have selected when you start painting will determine the maximum size of your shapes.  After some adjustment of the sizes and opacities on two layers, my background now looks like this:

My finished layout looks like this:

Happy scrapping!