This might be a very basic question with regard ABAC to which I didn't find an explicit answer What are the software projects artifacts, e.g. requirements specification, that are used at PAP to create the XACML policy documents?
The XACML standard and ABAC do not specify how to author policies. The Policy Administration Point (PAP) is out of scope of the specification.
From a technical standpoint, in Java, the easiest way to generate XACML policies is to use JAXB againt the XACML 3.0 schemas. This will give you a set of Java objects with which you can generate XACML policies. Other languages have similar techniques.
From a user experience perspective, you need to think about the policy authoring experience: how do you want to create policies? How do you want users to collaborate? Store policies? Deploy policies? Etc...
This is where different implementations (open-source e.g. ATT XACML or commercial e.g. Axiomatics) come into play. They offer different alternatives to author policies.
ALFA is also an interesting approach although it is not graphical but rather integrates with Eclipse.