The app comes with a number of stroke categories such as ink, pencil etc but you can also create your own via the brushes panel and one of the options is new round (right side menu). On the surface, this looks like it creates just a circular brush for use with the application but that is the start point.
You can select that menu command and the item is added to the current category so you may prefer to set up a category beforehand.
You now have your resource to work with, you can modify it by double clicking the entry in the panel and that will display the editor and you can then change a vast number of settings and luckily there is also a half decent preview of the stroke available, though you can also simply select the paint brush tool and apply it to the document and undo to try out the preset.
You can now go to the general tab and change the spacing, size, shape (very useful as you can then turn the artwork into an ellipse and then that is more useful with the rotation settings). You can change the accumulate and hardness and flow and that will make the preset more blurry or lower the opacity.
You can also add in jitter by going to the dynamics tab and adding in jitter for the size, accumulation, flow, rotation, scatter etc (vertically and horizontal as well as allowing for variation in the color (though that does depend on the current color selected). You can also modify them all using pressure, angle, randomness etc as well as changing the profile for each entry.
You can also add in texture as well as sub brushes such as adding additional round designs on top that can radically change the overall look of the preset. Literally 1000s of variations can be created by adding sub feature as well as textures and changing the settings etc
Sub,