On the command line
mvn install:install-file -Dfile=../DirtyMaths/target/dirtymaths-1.0.0-SNAPSHOT.jar -DgroupId=com.chrisnewland.dirtymaths -DartifactId=dirtymaths -Dversion=1.0.0-SNAPSHOT -Dpackaging=jar
Edit your pom:
<dependency>
     <groupId>com.chrisnewland.dirtymaths</groupId>
     <artifactId>dirtymaths</artifactId>
     <version>1.0.0-SNAPSHOT</version>
</dependency>