You first download Postal 1 and when it finishes you will either see an Postal 1 icon on your Desktop or you can manually open it from steam. If you want to open it from your Desktop, click on the Postal 1 icon 2 times with your left click. But if you want to open it from Steam than go to your Library and find postal 1, when done finding Postal 1 you should see a green button that says start, click on it. Thats it have fun.