How can I create a module to share between multiple projects?

Declaring a module only requires you to define a valid module descriptor. To be able to share it across multiple projects, simply move all components and the descriptor to a shared JAR. A module usually corresponds to a single JAR, but this is not a requirement.

See the reference documentation section on Development Across modules for general information on developing modules.
See also the question What is a good project structure for an Across module? for some best practices.