First, make sure that DOOM is installed on your PC through the Steam library. Launch the Steam client and sign in with your credentials. To view your list of games, select "Library" from the top menu. To find "DOOM," use the search bar or scroll through the list. Once you've found it, just click the title of the game to open its page. An "Install" button comes up if the game isn't installed. click it and wait for the download and installation to finish before continuing.

When you click the "install" button, if you want to see how long you have to wait, (I know your attention span is bad you little doomscroller go get a j*b) you have to click the text at the bottom of your screen that says "downloads".

Now that the game is installed, click the green button that says "play".
Well done. You have opened the best game ever!