Step one: Install nexus on a server in your LAN. It's excellent -- easy to install (really, just a couple minutes!) and solid. We have ~50 engineers and many CI servers banging on it all day and it's been stable for many months. Let's say you installed it on a server called "nexus.local" in your DNS.
Step two: Copy the settings.xml from http://www.sonatype.com/books/nexus-book/reference/maven-sect-single-group.html, fix the hostname as necessary, commit it to your source code system, and tell all your developers to copy it into their ~/.m2/settings.xml.
Step three: Set up your project's pom.xml properly. You'll want a "parent POM" that defines a "distributionManagement" section that looks something like this:
<distributionManagement>
<snapshotRepository>
<id>nexusSS</id>
<name>Nexus Snapshot Repository</name>
<url>http://nexus.local:8081/nexus/content/repositories/snapshots</url>
</snapshotRepository>
<repository>
<id>nexusRelease</id>
<name>Nexus Release Repository</name>
<url>http://nexus.local:8081/nexus/content/repositories/releases</url>
</repository>
</distributionManagement>
Step four: Enable "mvn deploy" -- go to your nexus UI ( something like http://nexus.local:8081/nexus ), click users, click "deployment", and give it a password. Then edit your ~/.m2/settings.xml and add this:
<settings>
...
<servers>
<server>
<id>nexus</id>
<username>deployment</username>
<password>PASSWORD</password>
</server>
</servers>
</settings>
Check that it works by running "mvn deploy", and you should have installed your project's artifacts into nexus.
Step five: Read this excellent documentation for maven: http://www.sonatype.com/products/maven/documentation/book-defguide