I built an app myself and made plenty of mistakes.

Handing out flyers actually sounds like a nice idea. Not because you'll get many users (you won't). But because you can ask for feedback face-to-face, which is more valuable than on a form or on a call.

If you want to promote the app, you'll need to share it online to reach a wider audience.

Before you start paying for ads, make sure you get the basics right. Define a market & strategy for distribution, launch on free platforms like Product Hunt, set up some Search Engine Optimization (SEO & ASO), and try your hand at a bit of viral marketing.

There are a bunch of free alternatives to Product Hunt that you can also try: Tinylaunch / Microlaunch / Simple Lister / Beta List / StartupBuffer / StartupInspire / Uneed

Source: https://speedbumpapp.com/en/blog/mobile-app-promotion/