Integration is the process of modifying a document type shell DTD to include different modules. It is also a purely mechanical process.

The first thing you should do is make local copies of all the DITA-provided shell DTDs, even if you don't change them in any way. This prepares you for the inevitable time when you want to add or remove domains or otherwise configure or extend from the base DITA types.

Note that the order in which the .ent and .mod files are included into the shell DTD is very important. This is because in DTDs, the order of occurrence of parameter entity declarations is significant. In XML, the first declaration of a given entity name wins, so to override entities defined in other included files, you must declare that entity first. Thus, you import the .ent files first, which provides the module-specific entity declarations that override the default definitions provided in the .mod files. Then you define the values of any configuration parameter entities, and then you include the .mod files, which use the configuration parameter entities to define the effective values for content models and attribute lists.