3) Download Patch 1.60 and install it. It will patch both games (ArmA 2 and Operation Arrowhead) so no need to worry about separate patches for each game or anything.
Installing the latest version of DayZ
4) Download worrom's DayZ Updater and extract the contents of the .rar to somewhere safe.
5) Go into the contents of the DayZ Updater folder, find the folder labeled "config", and the file named Settings.properties.
6) Open this file using Notepad or TextEdit or whatever, and find the line labeled "GameDir = C:\\Program Files (x86)\\etc etc blah blah"
7) Change this line to wherever you installed ArmA 2. By default it should be "C:\\Program Files\\Bohemia Interactive\\ArmA 2\\" unless you changed it yourself. Don't forget those two slashes at the end.
8) Hit save and exit Notepad. Now you can launch the DayZ updater and hit "Check for Update" or "Force Update". The only difference is that forcing the update redownloads and replaces all of the update files, just in case something went wrong last time you updated.
Making a shortcut to run DayZ
9) Go to your ArmA 2 directory, find the file labelled "arma2oa.exe" and make a shortcut to wherever you want to run the game from. This shortcut will be what you use to launch DayZ.
10) Right click on the shortcut, hit Properties, and at the very end of the line that says target, add the tags -mod=@dayz -nosplash
And that's it, you're done.
You can now safely and quickly run DayZ at any time by just using that shortcut that you made. You can update DayZ at any time by just running worrom's DayZ Updater and hitting Check for Update.