I talk to the person who knows the problem.
On a football booking app, I found out too late that what I was building and what the client expected had drifted apart. I should have checked much earlier. Now I show working examples early and often, so a gap shows up while it is still cheap to fix.
That means talking directly, not through a chain of messages. I want to see the current process, a sample document or the software you use today. For a new product, I want to know who will use it and why.