A good start is by turning on your computer, clicking on the power button, so that you have access to your steam and your game library

When you turn on the computer, look for the steam application and launch it normally

For this obviously you need to have the game purchased on your account, then go to the collection tab and look for the game with the name ( POSTAL 2 )
When you find the game you must click on it to download if you don't have it in your files and start

If you followed all the steps correctly, you should have your game open now, so just choose a graphics configuration that is comfortable for you and your computer and that's it, you can rip the head off of pedestrians who look at you crookedly.