You need to identify the differences between your new type of information and the more general type that you are specializing from. After you have identified the differences, you create a DTD file to declare the new elements that you require. Create another module to declare a set of mapping attributes for the new elements that point to the generic element types that they specialize. Then add import statements in the DTD file to bring in the mapping module and any ancestor modules. Finally, add a line that redefines the information types entity to include your new type. You now have a customized DTD.

When you specialize a domain, you need to first determine what elements must be specialized for the domain. Then you write an entity declaration file to list the specialized elements, along with their topic types and domain types. Next, you create a file where you define both the elements that are introduced for the domain and the specialization hierarchy. Finally, you write the shell DTD to combine the domain with topics and other domains.

These processes are described in more detail in the following documents:

•     Specializing topic types in DITA
•     Specializing domains in DITA