There is a PLoS Computational Biology paper (Ten simple rules for writing a response to reviewers) by William Stafford Noble (https://doi.org/10.1371/journal.pcbi.1005730) that contains excellent ten easy to follow "rules" (note that they are not rules per se). I will list the points from the paper here (and a one sentence summary) but would recommend reading this short article for more insight.
As with most things, take your time to go through these and formulate your own thoughts about it. While these are general markers that can help along the way, this is not a rule book (don't get misled by the word "rule") and not everything works/applies in all situations.
- Rule 1: Provide an overview, then quote the full set of reviews
Provide a summary of changes, along with new data, additional figures, etc, followed by interleaved comment+response.
- Rule 2: Be polite and respectful of all reviewers
"Keep in mind that if the reviewer failed to understand something, the fault likely lies, at least in part, with you for not making the point clear enough." - enough said!
"In general, even if the requested change seems unnecessary, it is usually better to go ahead and revise with the goal of showing the reviewer that they were listened to and understood."
- Rule 4: Make the response self-contained
Try, for example, a tactic like original and revision comparison style
- Rule 5: Respond to every point raised by the reviewer
"In some cases, the reviewer may disagree with your response, but you should not try to avoid a difficult point by simply ignoring it"
- Rule 6: Use typography to help the reviewer navigate your response
I find it easy to color code the comments along with keeping the comment in a
fixed width font while my responses are in regular Times New Roman black
- Rule 7: Whenever possible, begin your response to each comment with a direct answer to the point being raised
"Your goal is to show the reviewer that you took their comments seriously, and you should quickly convey what you did in response to their critique."
- Rule 8: When possible, do what the reviewer asks
"If the reviewer asks for 10 things, and you say that 9 out of 10 of them fall outside the scope of your work, then you are not likely to satisfy the reviewer."
- Rule 9: Be clear about what changed relative to the previous version
" In your response, refer explicitly to the previous and revised versions of your manuscript and explain what changes have been made."
- Rule 10: If necessary, write the response twice
"In practice, it is often helpful to write the "venting" version of the response first, wait a while, and then begin working on the "real" response several days later, perhaps after you have done some of the work to address the critiques raised by the reviewer."
With respect to fitting it all in a given word limit, try and see if multiple reviewers have similar comments/concerns; then you could answer them together rather than individually replying to all. Additionally, in this case, summarizing all changes in the response document will clearly not work.