Assuming you have a fresh Ubuntu 12.04 installed, and I assume you already understand what is the python script gem5.opt, gem5.debug, gem5.fast … etc if not you can easily visit the official website of Gem5 documentation http://m5sim.org/Documentation
$ sudo apt-get update; sudo apt-get upgrade
# installing the needed packages for Gem5
$ sudo apt-get install mercurial scons swig gcc m4 python python-dev libgoogle-perftools-dev g++ build-essential
# you can download the .tar.gz file and extract it but I prefer grabbing from the repo just in # case you needed to run $ hg pull to update the code or to commit
$ hg clone http://repo.gem5.org/gem5 # this is a developer version
$ cd gem5/
# Gem5 uses scons build system instead of make
$ scons build/ARM/gem5.opt
## To test the System Emulation mode you can run one of the programs shipped with Gem5
$ ./build/ARM/gem5.opt configs/example/se.py -c tests/test-progs/hello/bin/arm/linux/hello
# You can check out this live demo by Bayn in this youtube Video
Abdel Rahman Farag Hegazi 