Stupid Question: How do I write a research proposal in a new (to me) field?

I recently graduated with my Bachelor's, and am an incoming Master's student at Cambridge this year to complete Part III. I am also considering applying for a PhD afterwards, and I realise I will need to start applying in the coming months or so, involving coming up with a research proposal.

I am very excited to be coming to Cambridge this year and love the choice of modules they have available. I am particularly interested in learning more about some of these modules and could see myself leaning towards research in those areas.

My only issue is that I currently only have an undergraduate understanding (because I am a recent graduate) of the surrounding areas, definitely not post-graduate level. How feasible is it for me to both research a specific field, and identify gaps in research of sufficient quality to make a competitive research proposal when I am yet to encounter the subject in my graduate studies? I feel very restrained by the timeframe and how early I need to decide these things relative to my master's course.

That being said, I am aware that the answer may simply just be "do more reading", which I am currently doing. Any further advice on how to conduct initial PhD proposal research, or quickly bridge the gap between undergraduate studies and graduate research would be appreciated.

Thanks!