1st Step: Download the actual Version of UAC from the Official UAC Homepage.
2nd Step: Install UAC to a folder of your choice. Start "UAC.exe" or "start_uac.exe" out of your Installation folder, alternatively from your Desktop. If there is a new version avalaible, you will be informed now. You should see this window:
3rd Step: Enter your account data or register one new account. After a new registration, a pop-up will appair. It just tells you how long the activation eMail may last (roundabout 5 to 20 minutes). Be sure to check your spam directory.
4th Step: Press the 'play'-button on bottom left. You see your login with your unique ID, a checkbox called "enabled" and a list of games. Choose your preferred one. If your definition file for that game is out-of-date, you will be informed by a pop-up. Feel free to download the recent data. If your game isn't supported yet, you can try the 'generic' way. The MatchID isn't fix, you can enter anyting.
5th Step: When you're ready, check "activate". Now start your game. UAC now takes the first screenshot after 6 seconds (a countdown is shown), the following screens are taken randomly, at least one every 5 minutes.
6th Step: Now you want to see the screens. Exit the game and uncheck the "enable" box. UAC now uploads the whole screens to http://uac2.com/matches. You can either view the screens in your internet browser looking for the MatchID you've chosen or see them in your UAC. To do so press the 'screen' button. A new UAC window appears and gives you the possibility to search all the screens taken by game, user or matchID. You will be provided by all the screens from every user (if you look after them). At the same time, every user may find your screens.
Fixing some problems using Vista / 7:
A known problem is that UAC takes black screenshots or just catches your desktop. The solution is to disable the aero design.
1) Open your UAC directory (<...>\DexUS\UAC2)
2) Rightclick the uac.exe -> properties
3) Change to 'compatibility' tab
4) Check "Disable visual themes"
5) Check "Disable desktop composition"

Your copy of UAC should now work. For further support look at the official forums.