Step 1
Only one person is required to have this game, the other player can be from anywhere all around the world and doesn't need to own the game on Steam or even have Steam installed.
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Install a program called Parsec and tell your friend to install it on his/her PC aswell.
Link: https://parsecgaming.com/
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Note: this works long distance and minimal lag spikes (better internet will help tons though)

Step 2
Launch Parsec and go ahead and add your friend (top right has username)

Set up the options so it works to how you want it, you might need to play with the settings after you
invite your friend.

Your friend or you have to go to play tab, then i want you to hit co-play. your friend has to accept you in or you do the same for him.

Now connected double check if the settings are right, then launch the game.

Go to the local battles or co-op, at the screen or menu, your friend should be able to move their controller or mouse.

If that does not work go back on these steps and check settings.

If it works ENJOY you successfully done all the steps correctly!