A few months ago, I posted this: https://www.indiehackers.com/post/from-10-hour-smoke-test-to-4-500-emails-how-i-validated-a-saas-project-1d8435da12
Here's a summary:
I validated this product over the last 2 years.
It started with my wife needing to schedule text messages for work.
I then hacked together a website with Twilio and WordPress.
Through SEO, I got users.
Then I added a premium version.
I told my developer friends and they built TextSpot.
We're launching with 200+ subscribers and $0 in funding.
Check out the project at textspot.io