A really quick and simple way to do it if you're wanting something that looks pretty good is there's an app on android called thumbnail maker. It's free or you can pay to remove ads. Unfortunately I can't show you an example as I don't know how on the reddit app as I'm new, and I can't link to my YouTube which has plenty of examples as every time I try and link my post disappears due to the spam filter or whatever, but if you YouTube search "Borderline Jon" you can get some examples. This is not self promotion, I just don't know how else to show you.

The reason I like it is because it's really quick and straightforward and you can add lots of different types of amazing text. So if you're not experienced in photoshop etc you can pick a screenshot from the game and add large text. It looks pretty good for what essentially only takes a few minutes work!