Can anyone share a quick guide (not too comprehensive) to SEO for a first time Web app creator?

There's a lot of content out there to absorb. My recommendations for a web app, in a nutshell are:

• Learn the basics of on page optimization, know why title tags, meta tags, header tags etc are important
• Learn the basics of keyword research (know how to do research for high volume, low competition keywords)
• Find keywords you want to target for your app based on high volume, relative low difficulty
• Structure your pages and content (in particular homepage) to target these keywords
• Get your URLs right - understand what friendly URLs are, and target keywords with those as well (it's hard to fix URL related issues later in a web app)
• Put together a content strategy (normally blogging) to target long tail keywords and build content out over time

This was my go-to guide for many years, quite helpful: https://www.wordsinarow.com/seo.html

Lately I've been using this one as a reference, it's a little more up to date (though it looks like the first one was recently updated): https://moz.com/beginners-guide-to-seo

I don't have a "step by step guide" which it sounds like you're looking for - but on the most recent IH podcast the same question was posed to Nat Eliason. His response:

"Try to make sure that you're not overly complicating the process. I would say, if you want to learn about SEO in general, I think you could basically just read Brian Dean's blog and watch his YouTube channel and you wouldn't really need to go anywhere else. If you are curious about it as it blends with content marketing I would very selfishly promote the Growth Machine blog, because we do write about it a lot and I talk about it a lot too."