DaiZy is a mod that allows you to play single player DayZ offline. It has all the same features as the regular online version, including friendly and bandit units (controlled by AI), loot, vehicles and everything else you could possibly want. It's also allows cheating through the additional installation of one or two other files.
Before you start, you need to have DayZ installed. If you haven't got that up and running, go off and read this ridiculously detailed guide that over 800,000 others have used. Go on, I'll wait.
Once you've got DayZ installed it, you can just follow the video above. The files it downloads come from here:
There's a more detailed written installation guide to be found here:
And here's a guide to installing Loki's Lost Key (that's the cheats thing):
There are other variation of DaiZy and support for other maps. Check our forum (embedded below or via the link at the top of the page) for more details of those.