How to use the Photoshop brushes to create unique custom shapes by using selections / image trace etc tutorial
2) Apply single dab or multiple dabs as required (perhaps apply as a brush stroke)
3) Select the stroke using the selection tools (magic wand tool perhaps)
4) window menu and paths
5) Make work path
6) edit menu and define custom shape
7) Shape can now be saved via the custom shapes panel
An alternative approach and probably better would be to export the Photoshop brushes design via the pasteboard or via the export of a PNG image and re-edited the design in a vector application and use an image trace tool to create a brush path based on the created brush dabs on Photoshop. The vector trace can then be imported into the application via the pasteboard and saved as a new shape.
Personally, I prefer tools such as Corel Trace for the vector trace of artworks and there are also many online vector trace tools (some that you have to pay for) but with a little tidying up and a little care, decent results can be achieved with the trace feature in Illustrator (though I would love to see a proper overhaul of the tool to include a variety of different ways to do the trace). Another alternative can be found via the Stipplism plugin (for Illustrator) from Astute Graphics though not particularly a trace tool, the results can be very impressive in creating all kinds of sketched and rough variant traces from Photoshop brushes depending on the symbols etc used to create the trace and then from there the results can be re-imported into Photoshop and then defined as a new custom shape / vector design.
You can find many additional tutorials on Photoshop such as how to combine multiple gradients in Photoshop CC 2017 2015 etc and useful notes such as how to use Photoshop brushes on layers and the exporting of resources in the application such as how to export the brushes for use in other applications such as PSP and color tutorials such as how to use the Photoshop brushes across channels and utility notes on the application of the vector shapes and how to use the Photoshop custom shape panel options like constrain and center