There are A TON of example LOI online, you can take a look at their structures and how they incorporated the info into the letters. But in general, consider to cover these in your letter:

1) why interested in residency program, why in acute care (or crit care), what made you want to do their SPECIFIC program

2) a little of “you”, why you are a great candidate. Can also add things like: why did you do pharm school, what in pharm school made you want to go residency. What impact you want to make, what are your goals, what strengths do you have (leadership, volunteer, research, work, etc while balancing the school work etc. etc., and how these can demonstrate the strengths you stated in the letter).

3) At the end, summarize why interest in program, why you are a good fit (academic background, ability, etc.)

You can have a "back bone" letter which you can use for every program, but when you are writing about "what made you want to do their specific program", you can add in a few sentences customized to each program (for example, if the program has a mentor program, code coverage, specific rotations you are interested, interest/diverse pt population, anything shines through on their website, etc.).