The first thing to do when coming across such a question, in my mind, is to vote to close as too broad. Sorry, but as posed (and I have seen a few such examples myself, so I know what you mean), it's simply asking for far too much. It cannot meaningfully be answered in its current form, particularly within the Stack Exchange format, and therefore should be closed. Remember the so-called book test: if you can imagine an entire book that answers your question, you’re asking too much.
However, voting to close should only be the first step (first in order to ensure that people don't waste time answering what amounts to an unanswerable question).
The very next thing to do is to leave constructive criticism in comments. Such should ideally always accompany a vote to close; in practice, I know that it doesn't always work out that way. For example, particularly for a user who is new to the site or the network:
- If you know of any similar, relevant questions, then link to those. If space allows and it isn't obvious from looking at them, discuss briefly why you feel they are relevant. Avoid calling them "duplicates" if they aren't actually; I like to use "you may be interested in..." or just plain "relevant:" especially if space is tight.
- Explain why the question as posed cannot be answered within our format.
- Try to guide the person asking the question to narrow it down such that it becomes answerable.
- Point out that having a question be put on hold is not the end of the world, and that if the question gets edited during the "on hold" grace period, it is automatically nominated for the community to review for reopening.
- If you can identify some aspect that would narrow the question sufficiently to bring it into scope, then suggest that the OP edits the question accordingly. Do not, however, make such an edit to the question directly, at least unless and until the OP has clearly expressed that they are happy with it; doing so would likely be changing the intent of the question's author, which is something we should always strive to avoid as much as is at all possible.
- Do point out that it's okay to ask multiple questions about a single subject, but that they should be asked as separate question posts and that what one learns from one should be incorporated into the next somehow. (Asking multiple, related questions in a single post is an art that takes time to get the feel for. It's very easy to take what could be a perfectly fine question and turn it into one that is too broad by doing it.)
- Point the person asking the question at the site tour (
[tour]) and/or help center (
[help]), as appropriate. Particularly if pointing them at the help center, don't be afraid to direct them toward a specific section or even a specific article within the help center. Those texts aren't perfect, and for the very most part they aren't tailored per site, but much of what's in them is generally applicable.
- If the asker appears to be receptive, pointing them to Eric Raymond's How To Ask Questions The Smart Way can also be beneficial; it even has a specific section for Stack Overflow. As written, How To Ask Questions The Smart Way is tailored mainly for technical questions, but lots of it applies equally across subject disciplines, and I tend to find that following the general advice laid out there usually gets me useful answers in a variety of fora.
The reputation limit to participate on Meta has been lowered to 1 on Worldbuilding, so pointing them to the question sandbox is probably a good idea.
If the asker has sufficient reputation to participate in chat (20 rep), then pointing them to our main chat room can be an option for more back-and-forth discussion about the question.
Unfortunately many of the people who most need help getting their questions figured out are those with the lowest reputation. However, I am personally very much against "sympathy upvotes". Posts should be voted on based on their content, not based on who or the reputation of the user who posted them. Instead, encourage them to go answer some question that doesn't require clarification. A good answer to an existing, even an older, question can quickly earn someone several upvotes, moving them quickly past many of the new user reputation limits.
Generally speaking, I try to be more gentle with new users. Someone who has earned a few hundred or even a few thousand rep on a site, particularly with a history of posting well-received material, should know how the going goes; particularly so if they have access to cast close/reopen votes and review such votes, they should be familiar enough with how the system works that they shouldn't need specific guidance. In such a case I might leave a brief comment and maybe link to some other page or question if I have it handy, but I won't spend significant amounts of time hand-holding the person asking the question or digging out obscure posts. More or less the same goes with people who have an established presence on the Stack Exchange network, but who are new to Worldbuilding SE specifically; I might cut those a little more slack for not being familiar with the specific standards of the site, but not for very long. Basically, if you have earned enough reputation elsewhere to get the association bonus to start out at 101 rep instead of 1 rep, I expect that you know how things work at least to some degree.