Article: There are a few tabs at the top of the Steam client, one of them is "Library." Click it, and a drop-down menu will appear. "Games" is the first item on the menu. If you have any existing games in Steam, it will all be listed on the left panel. On the bottom left-hand corner of the app, click "Add a Games" and select "Activate a Product on Steam" from the menu that appears. A dialog box that will guide you through the process of adding your AoE2HD product code. The dialog box will prompt you to enter your game's product code. This code is provided to the buyer when purchasing the game and will usually be found on the retail CD/DVD case that came with the game. The code is not of specific length and can contain both alphabets and numbers. Enter the code into the fields and follow the onscreen instructions to complete the process. Note that, because using the code to install the game on your computer and adding the game to Steam are two separate things, it is still possible to use the code on Steam after you've already used it to install the game on your computer.

What is a summary?
Access the Steam Game Library. Select "Games" from the drop-down menu that appears. Add your AoE2HD product code to Steam.