First thing you need to do is be able to access the console. To do this you first need to make a shortcut on your desktop to launch halo.

Second you need to right click on it and bring up the properties.
You will need to add, (-console) at the end of your target line, do not type the ( )
So it should look something like this "C:\Program Files\Halo\halo.exe" –console
Your target line might vary depending on how and where you installed it, don’t change anything just add –console

Now launch halo buy double clicking your shortcut, when in the game hit the ~ key and type your commands.