For a Research Assistant, being a Pharm.D graduate
While I don't have first hand experience with medical writing, I'm speaking from my experience as a writer of educational content for researchers - the most important thing for anyone wishing to write scientific papers ( whether for themselves or on behalf of others) is to read the available literature. A medical writer would need to familiarize themselves with different types of articles published in medical journals. The medical researchers who usually employ the services of a medical writer are super busy doctors who would not want t spend too much time explaining what they expect you to produce. You'd need to be familiar with academic writing, creating tables and figures, understanding how to write a case study versus how to write a meta-analysis or document a clinical trial. This will ensure that you quickly understand what the researcher wants and how to write a manuscript based on the data they provide.
Here are a couple articles I had written a few years ago which you might find useful to start off with:
Thanks, for the advice inclusive of noteworthy information @Kakoli Majumder. I found this useful.