You need to build the jar and then add it to your libs folder in your project.
How to build the jar:
bash
$ git clone https://android.googlesource.com/platform/frameworks/volley
$ cd volley
$ android update project -p .
$ ant jar
Here is a YouTube video if that would help: https://www.youtube.com/watch?v=5bwn7DYt2AI
You can also download a non-official build on maven central.
Edit:
It looks like volley can't be built with ant anymore (correct me if I'm wrong). All I did was create a new Android Project, mark it as a library project, create package com.android.volley, copy the source over to the project, and build it.