"How Can/Should I Do This Thing With My Story/Setting/Character?"

First of all, make sure you know exactly what it is you want to do. For example, let's say you want to write about a scientist. The question isn't "how do I write a scientist," but rather, "what kind of scientist do I want to write?" Do you want your scientist to be perceived as smart? Ethical? Are you aiming to write a fantasy kingdom? What do you want it to be like? Do you want it to be inspired by the Medieval period, or do you have something else in mind? Knowing exactly what it is you want helps you ask the right questions to find the answers you need.

Ask yourself what could work here or what might make sense. Think up as many things as might work or make sense and write out a list. Quite often, this is the only thing you really need to do. If you're worried that what you come up with is too commonplace, you might take a look at "Help! I'm Worried That My Idea Is Too Cliche!" - What To Do When This Happens.

If you're trying to create or write a character, ask yourself what this character would know or have seen, and research that. For example, a scientist would know about the scientific method and how to conduct a double-blind study, so that's a place to start. A scientist would also likely know what to call a lot of scientific tools and equipment (especially the ones relevant to the scientist's own field), so you could look into that, too.

Ask yourself what happened to produce your situation or result. Nothing simply just happens, so stop and think out what lead up to the circumstances you want to write. Is there a rebellion going on? What made people feel desperate enough to rebel? What made them think a rebellion would work? Are you writing a character who changes sides? What all happens to make that character think this is the right choice? Write out every step and every factor behind it. If you can't actually come up with the answers, it can either mean you either need to research more, or that your idea isn't very believable.

Remember that multiplicity lends credibility in scenarios. In real life, things rarely happen for one reason and one reason alone. Try to come up with at least three reasons or factors behind a big decision or a big event. A benefit of this is that even if one reason might seem fairly weak or questionable to someone, there will be at least two more to back it up.

Look at real life cases or equivalents. If you're struggling to come up with answers out of your own head, do this. Even if what you're writing has fantastic elements, researching the real life counterpart can still give you a pretty solid foundation to build on and help you give your world verisimilitude. A few examples of resources you might look into include documentaries, history websites, and educational videos on YouTube. You might also think back to your own family, friends, schoolmates, or co-workers - did you ever see anything like what you're trying to go for happen with them? How did it go? How did it ultimately end up?

Look at or think back to similar media. How did other stories do it? Which aspects did you like, and which aspects didn't you like? What would you change to make it better? And what have you seen that succeeded in creating the kind of emotions or atmosphere you want to capture in your creation? What did it do that made it work? Can you perhaps take inspiration from this? (A warning - it's important to be careful that you don't end up copying other works' ideas wholesale. Borrowing & Sharing Ideas In Fiction - When It's Okay, & When It Isn't has more information on this.)

Make a habit of paying attention to things you don't need immediately. If, for example, you see an interesting fact on how old-timey weapons work, make an effort to remember it. Take some time to simply read an article on toxic relationship signs or take a look at a how-to blog. If you see something happen in real life that evokes a particular emotion or would lend to a dramatic scenario at some point, take note of it. You might not need this information right now, but you may find that it becomes very relevant in a future project.

Remember that sometimes, the answer is just "whatever you want." What should your fantasy kingdom be like? How should magic work in your setting? Should you write hard sci-fi or soft sci-fi? Should the vampires in your story burn in the sun or not? In cases like these, the answer is usually "whatever you want," so aim to figure out what it is that you really want, and do that! If you're having trouble with this, you might take a look at How To Make A Decision When Choosing Is Hard.