Gustav The habit, in one sentence: stay in constant contact with the people who pay you, and build from what they actually say instead of what you assume.
Why it matters
Left alone, you will build an elegant solution to a problem nobody has. It happens to smart people constantly, because the inside of your own head is a very convincing place. Customers are the correction. They tell you which feature matters, which one nobody noticed, what the real bottleneck is, and what they are willing to pay to make it go away.
This is also where your next work comes from. The client who is happy with one project will tell you about three more problems if you simply keep the conversation open. Silence after delivery is a missed pipeline.
What it looks like in practice
On every real project I have built, the useful direction came from the client, not from me planning alone. Putting an early version in front of them surfaced needs I never would have guessed. The booking and email work I have done evolved because the people using it said "this part is annoying" and "could it also do this", and those sentences were worth more than any roadmap I could have written by myself.
How to build the habit
Make contact routine, not occasional. A short check-in during a project, a message after delivery, a real conversation a month later. Ask open questions: what is still taking you too long, what almost made you not buy, what would you want next. Watch people actually use the thing if you can, what they do tells you more than what they say.
And talk to the ones who said no. A polite "what made you go another way?" is some of the most honest feedback you will ever get.
The pitfall to avoid
Do not ask questions that only invite politeness. "Do you like it?" gets you a yes that means nothing. Ask about behaviour and money instead, what they actually did, what they would actually pay. And do not delegate all customer contact away the moment you can afford to; the founder who stops talking to customers slowly stops understanding the business.