Q: How to write a research question?
I am not sure if you mean ‘how to arrive at a research question’ or simply ‘how to write or draft a research question’ once you have identified the problem you wish to research.
To arrive at a research question, you need to explore in detail the research area you wish to study. Go through recent literature in the area to identify gaps or suggestions for new directions. These are typically indicated in the Discussion section of the research paper. Once you have done the background work, choose a question based on the following aspects: feasibility, value or relevance (for the target population), and interest or motivation. Interest or motivation is important as you need to choose a problem that will keep you charged through the length of the research and publication.
Writing or drafting the question becomes easy once you have done the background work (described above). You may need to frame the question in a few alternative ways before you can decide on the final question. However, also be open to the fact that you may need to modify the framing of the question, or even the question itself, after commencing your research, based on facts or observations that you uncover as you go along.
Here are some excellent resources to help you arrive at and write a research question.