The format of qualifying exam varies among schools, programs, subject matters, and advisors. So it is hard to give solid advice on the matter, but I will share my experience with you. I took my qualifying exam in PhD in cell biology a few years ago. I had to write a grant proposal which needed to be handed in two weeks before the oral exam.It was not a smooth road, mostly because of my difficulty writing. But on the oral exam day, I was pretty comfortable.
Qualifying exam committee:
My qualifying exam committee consisted of 6 people including 3 people from my department and 3 people outside of the department. I picked my committee based on my experiences with professors and got recommendations from my advisors. My advisor was helpful to guided me through this. It is important to make sure these people have heard about your research at some extent and comfortable with it, otherwise it could potentially make your exam process miserable. Some people who have no clue about your field, they either might be the quite ones who won't say much or ones who will give you the hardest time. So it is important to make sure everyone in your committee is comfortable with your research.
I have never written a single grant proposal before, so it was very overwhelming. The grant proposal writing class my school offered was useless at the time (since then they have changed the structure). My advisor was not very hands on during process. Writing is my weakness, so I had a hard time. I would say just make sure to pick simple experiments for your own sake and write it clearly. If you have trouble, you can always meet with your qualifying exam committee members for advice, which I did not know. So don't be afraid to get help. The important part that I did not realize at the time was that they are not looking for a good grant proposal from you anyway, since it is your first time, so good effort and logic will be suffice.
I thought my oral exam was solely based on my grand proposal. I was so wrong. They could care less about what I wrote (as I said before they are not expecting much from the proposal). But it might really depend on who you have in your committee and how your program runs their exams. So I would ask fellow graduate students in your program and your advisor for general guidance on how things go in the oral exam. My exam questions were mainly on foundational knowledge on my research field and research methods. The point of the oral exam was to make sure that I understand my field and have basic research methods to design an experiment knowing weakness and strengths of options available.
That was really it for me. I was so stressed out during this process and even doubted my ability and choice, but clearly I was overreacting. If you find yourself stressing out, just make sure to take a breath and believe in yourself! If you have any questions, feel free to comment or email me. I hope it was helpful.