Creating a modular project is just as easy as adding a topic to a project. Once you have added the child project its TOC is displayed as part of the current project's TOC and you can edit it directly if you turn off read-only mode (see below).
When you publish the master project the entire TOC of the child project is inserted in the master TOC at the point where you insert the child project in the master TOC tree.
See Modular Projects in the Reference section for some more background information on modular help systems.
By default child modules are inserted in read-only mode. This is recommended for multi-user editing, otherwise all topics in child projects will be locked for all users. You can turn off read-only mode in Configuration > Common Properties > Miscellaneous.
Modular Projects (Reference)