This should give you a gist of how to use a directory inside your project as a maven repository for libraries that aren't published to a public repository.
As written, this will point to the [project-root]/lib directory, which should be formatted in standard maven repository format. You'll "install" new libraries to this directory using the mvn install:install-file
task.
The examples below assume we'll be installing shawmans-lib-0.1.2.jar version 0.1.2 with groupId com.shawmanz32na and artifactId lib (determined from the source, which is structured like src/main/java/com/shawmanz32na/lib/code.java).