Here are 3 ways to get started (see screen shots below):
1. Take different media and make some sample marks. Scan them at 300 ppi, grayscale.
2. Get some photo samples. I selected some irregular, textured ones.
3. Make your own marks in Photoshop. In the example below I spaced out some dots (1 to 3 pixels, solid black) to get a brushstroke effect.
Note: You can also use text that you created with your Text tool.
With all of these approaches, make sure to go into Levels (cmd-L) and slide the Black Point to the right so that you have lots of areas of 100% black. If you don't, then you can't paint 100% of the Foreground Color with the Brush that you generate from the original shape. It's fine to have some gray areas.
|1. Scanned textures that I created traditionally.|
Click any image to enlarge.
|2. Photographic textures.|
|3. Bristle textures that I created in Photoshop.|
Once you have your shape, use the Lasso to select it. The shape that you Lasso is important, because it will be your Brush shape. For the photographic textures (#2 above), you may want to erase away so that it leaves an interesting shape to create your Brush from.
|Draw around your Brush shape with the Lasso.|
With the Selection still active, go to Edit (Menu) > Define Brush Preset and give your new Brush a name. I usually begin the name with "My" (like "My Bristle Brush") so that I know that it's one that I created.
And congratulations! That's the first big step in creating your own Photoshop Brush. When you go to the Brush Presets (with the Brush active: right-click, control-click; or Brush Presets in Menu or Brush Panel) it will show up at the end of the list.
One important thing to realize is that you get very different results when you take a Brush generated from a textured source (like the photographic textures above) and click-and-release (like a "dab") versus click-drag-release. The "click-and-release" method preserves the texture, the click-drag-release doesn't.
Of course, you're not finished. I'll cover using the Brush Panel to modify your custom Brush and to add effects (and to fix the Spacing issue).