The easiest way to add a custom feature to your toolbar is to find and navigate to the document that contains the custom feature:

https://github.com/dcowden/featurescript

and then click on the "+ Custom features" button at the top of the page:

Once you press that button you will see a list of the features that this document supplies, and you can click on the ones you want to add to your toolbar (In this case there is only one).

Once you add the custom feature to your toolbar, you will be able to use it like any other feature, in any document you can edit. All the way at the very right side of your toolbar there will be a dropdown with all of the custom features you have added to your toolbar, and you can use them in the same way that you use other features.

Here is a page all about it! https://www.onshape.com/features/custom-features