Pixelmator Pro brushes are composed of two images (or textures) — a shape and a grain. When you paint, the shape and the grain are repeatedly stamped onto your image to create continuous brushstrokes. You can think of the shape as the outline of a brush and the grain as its internal texture (although the shape also contributes to the texture of a brushstroke).
To create your own brush, you’ll need images of the shape and, optionally, the grain. When creating an image for the brush, any white areas will be 100% transparent and any black areas will be 100% opaque. So a black circle on a white background would create a regular round brush.
Once you have a shape, you'll need to duplicate an existing brush to create a new one. If you're aiming for a look similar to that of an existing brush, it might be best to duplicate that specific brush and start by replacing its existing shape texture.
Choose the Paint tool by doing one of the following:
At the top of the Tool Options pane, click the name of the brush to open the Brushes Browser and select a brush.
–click a brush and choose Duplicate from the menu that appears.
After duplicating your brush, you'll need to replace the brush shape and texture.
Alternatively, you can simply drag in a new texture directly from your desktop, Finder, or other locations.
Once that's done, feel free to edit the settings of the brush and make it paint just the way you want.