It's a big question, but I'll try to answer it as succinctly as possible. As someone who has been a UI/UX designer building other people's software and now CEO of my own SaaS company, I've learned over the years that it is never a straightforward, linear process, but do the following:
1. Once you have your basic idea, start by doing a lot of research into your target customer and competitive analysis, to really understand where your app fits in the market. Ideally you want to have at least a couple other similar products but you should have an idea of where they fall short and how you can be better. Talk to 50-100 of your target customers and ask them about how they currently solve the problem. Your goal is not to sell your solution, just listen and take notes.
2. Once you have your idea and you understand the 2 or 3 most important features that make up the core of your product, design the UI and prototype it using a tool like Invision that lets you quickly send it out for feedback. Then use a tool like UserTesting.com to watch people use it and get more feedback. Send it to the people you already interviewed and get their take on it. You should uncover some holes that need to be fixed at this stage.
3. Start coding your MVP, but remember - keep it minimal - Don't spend longer than a couple of months on it because it's going to change 1,000 times anyways. Just get something quick and dirty into people's hands for more feedback.
4. While the MVP is being built, you should be marketing it. Gather emails from a landing page, start blogging, get some media coverage, build some buzz. You shouldn't expect to make a lot of money from the early launch, but the key is to get a list of people (100-1,000) willing to use the product and give you feedback, then keep building on that foundation.
5. After the MVP launch, spend as long as it takes (8 months+) interviewing beta users, refining it, rewriting it, doing whatever it takes to get to product-market fit. While doing this keep building up your email list and get a steady stream of traffic through SEO/social/content but don't spend any money on ads until your at or close to PMfit.
Hope that helps! This is what I did with Proposify (and stumbled a few times) and it worked. We hit PM fit and within one year went from 30 paying accounts to 1,000+. Book a call if you'd like to learn more.