You can redeem the Steam Game codes through the Steam Website or the Steam desktop application. Here are the steps you can follow:

Through the Steam website

1. Visit the Steam Product Activation page (Make sure to log in with your account first)
2. Enter the code you have purchased in the box, check the Steam Agreement check box, and click the "Continue" button
3. Follow the instructions until the activation is successful

Through the Steam desktop application

1. Open the Steam application and log in to your Steam account
2. Click the "Game" menu and choose the "Activate a Product on Steam" option
3. Click the "Next" button
4. Read the agreement and then click the "I Agree" button to continue
5. Enter the code you have purchased into the box and click the "Next" button
6. After the activation is successful, click the "Finish" button. Then you can proceed to your Steam game library and install the game that you prefer