Sorry, but I hate when people say this to indie developers like it's some grand new idea to them. Marketing is hard. You can't just post about it online as many places have self promotion rules. You can try to reach out to get reviews, but chances are you won't get noticed. Everything else costs money, and if you don't know what you're doing (and chances are the indie developers doesn't) you're just throwing your money away.

Its not a 1:1 comparison, but I relatively recently released three extensions for Photoshop. They're well made and designed, unique, and would be a boon to a lot of photographers out there. I contacted every place I could find and got reviewed at only one place, Fstoppers. The reviewer really liked the extensions. I got a whopping $1,000 of sales from that and my many months of development time. Frankly, I didn't have a clue where to go from there. I'm a wedding photographer and I had to release them before my wedding season got going. Now that the season is over I still don't know. I've been running Facebook ads but without being able to target professions I haven't been able to work out a strategy that works.

Long story short, marketing is hard and takes both time and money.