Step 1

Create a field set on the Case object.
 
Navigate to Setup--> Customize--> Cases--> Field Sets. To create a new field set, simply click New, specify a name for the field set, and drag your desired fields into the field set. Once completed, click Save.



Step 2

After step 1 is completed, you now need to create a new button that references your new field set for Case Split.

Create a new button on the Case object object. The properties and URL of the button should be similar to the image below.

The field set parameter will need to be adjusted to the API of your custom field set. In this example, our field set is named "CS_Fields".

You may copy and paste the following URL, substituting 'fieldsetname' with the name of the field set to be used with the button:

lightning/cmp/csplit__CaseSplitWrapper?csplit__caseId={!Case.Id}&csplit__fieldSet=fieldsetname 



Step 3

Add the new Split button to your page layouts.

Navigate to Setup--> Customize--> Cases--> Page Layouts--> and Select Edit next to the Page Layout you wish to add your new button to. Locate your button using the menu on the top of the edit screen, and drag/drop it onto your page layout. Finally click Ok, and Save.