[Update]: Slime-covered yet adorable Slime Rancher 2 is heading into Early Access on September 22, 2022.
The Epic Games Store released a little while ago, and in order to entice people to come over and check it out, Epic announced that they would be giving away a free game every two weeks throughout the entirety of 2019. The current offer, as you might guess from the title alone, is the rather adorable and quite quirky 'farm simulator' Slime Rancher.
I haven't played Slime Rancher myself, so I can't offer you much in terms of advice, but from what I've seen it has generally positive reviews across the board. So if you're a fan of the genre, or just looking for something to relax with, Slime Rancher does appear to be worth a closer look. After all, it is entirely free!
As for the process of acquiring the game itself, it's pretty much as simple as it gets. Just create an account on the Epic Games Store website, go through the 'purchasing' process, and you'll be able to play Slime Rancher whenever you want. The one catch is that this promotion will only last until March 21st, so do make sure to grab yourself a copy before it goes back to its normal price.
And as a final note, allow me to leave you with one of the trailers for Slime Rancher. Have fun herding slime!