Step 2: If you’re a VIP member, it will show a separate “VIP” tab on the top of the screen.

Step 3: Go here to grab the voucher code:
https://www.greenmangaming.com/vip/vip-deals/

It will say:
“Just for you - Save an extra 15% on the Holiday Sale.” with a green button just below it.
You do not want this one. Instead, scroll all the way down to the bottom of the page.
There you will find another green button that says “GET YOUR CODE” which you will find next to the “Mystery Sale” icon.

Step 4: Click on the green button and it should reveal a voucher code that looks like this:

Code:
#00E260-NGV12E-SSS2L9
(The voucher code is unique so do not copy & paste the example provided here)

Step 5: Now go ahead add the ‘Mystery Game’ to your shopping cart.

Step 6: When you proceed to the check out process, you will see a section where it says “Apply voucher code” or similar. Copy & paste your voucher code from Step 4. Make absolutely certain that when you copy & paste it, you include the hashtag symbol (#) exactly as I’ve shown in the example otherwise, you’ll get an error message saying ‘sorry, your code is invalid’. If you’ve done it correctly, the price will drop from $1 to $0 (FREE).

The section where you enter the voucher code will appear BEFORE you you get to the payment/credit card information screen so if the website is asking you to enter your payment information, you’ve gone too far and you need to back up. I did not have to enter any credit card information whatsoever when I did this.

At the end of the check out process, it should spit out a game key which you can activate on Steam.

List of possible games that people have reported thus far:

Hospital Tycoon
Last Inua
Operation Flashpoint: Red River
Uncanny Valley
Overlord
Operation Flashpoint: Dragon Rising
Overlord II
Damnation
Maelstrom
Normality
Spermination
Club Manager 2016
F1 2015