With Work Relay API checkbox checked, you are offered to choose a class and a method from the API, and the list of parameters you should enter to run the method:

To call your custom Apex, put those calls inside of a class that will implement one of the Work Relay interfaces, WR_BPM.WorkflowCallApexActionInterface. Here's an example of such class:

public with sharing class WorkRelayApexCalls implements WR_BPM.WorkflowCallApexActionInterface
{  
    public void run(sObject context, WR_BPM__Flow_Instance_Cursor__c flowInstanceCursor, String operationType, String parameter)
    {
        Map <String, String> params;
        if(parameter != null) params = (Map<String, String>)JSON.deserialize(parameter, Map<String, String>.class);
       
        if(params != null && params.get('callType') == 'doSomething')
        {
             String paramToDoSomething = params.get('paramToDoSomething');
             //your code for doing something    
        }

        else if (params != null && params.get('callType') == 'doSomethingElse')
        {  
             //your code for doing something else
        }
    }
}

This example implies that you are unifying usage of Work Relay calls of your custom code inside one class, WorkRelayApexCalls, but whether you want to have it this way or create multiple classes for logical separation, is up to you.

Parameters received by the run method are:

• sObject context: process instance's context record 
• WR_BPM__Flow_Instance_Cursor__c flowInstanceCursor: cursor that "owns" the step in the instance
• String operationType: Proceed/Approve/Reject
• String parameter: whatever parameters passed from the action as JSON.

The screenshot below illustrates how this class's call is made (7) from the Process builder:

Check Execute in background checkbox (8) whenever you need to perform this call in asynchronous manner.