I’m writing my first recommendation letter for a student (for grad school) who is excellent. I’d like to, thus, write a very good recommendation letter. Are there informal guidelines for writing recommendation letters so that my letter does not come off as insincere and reflects the student’s quality?
If you can write about what the student has done, rather than just the grades in courses, it is a plus.
Any special projects? Any leadership actions? Are they self directed? Do they have insight? Are they helpful to others? Have they done independent study, alone or in small groups?
Would you take them on for a research project without qualification? Can you confidently predict their success, in school and beyond?
But itemize actual actions when possible.
Think about how you know the student is excellent -- what sets them apart. In particular, if this is the first recommendation you've written, that suggests you may have a limited basis of comparison (to other students you've worked with). The difficulty you have in conveying your opinion may be related to an underlying difficulty in justifying your opinion.
A useful guideline is show, don't tell. Here, this means it is much more convincing to cite distinctive, concrete, factual observations about the student than to pile on generic superlatives. For example you could note if they are going above and beyond course objectives, are doing research-level thinking, or are "the only student in our department to accomplish X in the last N years".
Quantify their rank
I'm in a hard science field, so the answer may be less relevant to humanities letters. I have found that the strongest letters highlight specific interactions and anecdotes about positive qualities, but most importantly rank the student relative to a distribution. For example
I have been teaching Physics 101 and Engineering 305 for the past 4 years, and this student is easily among the top 5 of the ~200 students I have taught.
If you've never written a recommendation letter before, you probably have a smaller pool to compare against. In principle that might weaken your letter some, but really that's important information to your reviewer. A letter ranking the candidate top 5 out of 20 years of experience vs 2 years is legitimately more weighty.
The main thing to remember is to explain the basis for your evaluation and give a sensible comparison of the student relative to the cohort. It is not necessary to give detailed information on grades (since that will be given separately), but you should give some underlying objective information that explains your assessment that the student is excellent. What have they done that excels relative to what you expect for students of that cohort? What skills does this student have that other students in the cohort do not have? Etc.
You could always emulate the style of Richard Duffin when he wrote a recommendation letter for John Nash.
I think it's important to show and not tell. While most people simply use adjectives to describe, it would be great if you can show concrete examples, past interactions etc that you had with him/her, which can drive your point across better.
Do also say, what difference did he/she make. What could not have been done without their attributes ?