Best tip I can think of is you should allow the project to grow organically. If the project feels like it naturally wants to tell this story or that story then allow it to go in that direction. You can try too hard to control a project and it will feel robotic. But, if you allow the project to find it's own path it's a lot easier to focus the story it wants to tell. You'll know it's the right story because it'll feel right.
You can go into it wanting it to look a certain way or to have a certain something to say, but it doesn't have to say anything if it doesn't want to. The viewer will watch and be their own judge, or your project can be decidedly about something and you want to prove your point and move the viewer to agree with you. But if you are at a point now where you don't know what the story is, then it will come to you during the interview process. Then you can start turning the way the docu goes in that direction by engaging the interviewees with appropriate questions and conversations and editing the film in a certain way to support those ideas.
But if the story wants to be blue, and you want it to be red, the final docu might be red...or it might be purple. But it won't feel as right as it would have felt had you just allowed it to be blue.
It's great to get in there and talk to a printmaker and want to do a film with them. Maybe it becomes about her grandparents that inspired her to make prints, or it's the story of the 200 year old print machine he uses, or it's about...whatever. There's a story in there because everyone and everything has a story that wants to be told. It's your job to either listen to that story and tell it to your audience, or to ignore the story and tell your own vision through the apparent subject of the film.
Either way, in the end, it's your film.