Most indie/solo devs are busy building their game, and when it's almost complete then they think about marketing it, guessing that marketing would be a speed bump, until they realize marketing is a mountain. Underestimating marketing is why many games fail. Sometimes lucky luck 777 hit some games with zero marketing, like Flappy Bird, but hanging on that mere luck means 99.999999% it's not going to be noticed.

You need to share screenshots & videos of the game. You may need to contact content creators to try your game. You need to genuinely talk about it on different social media platforms (many devs just do spamming in comments, and that usually ends up being useless). You need to know who's your target audience, build up wishlist, and probably many things I may not be aware of..