Bookstagram(Instagram profiles who only post about books). Create a post about Dear Martin and write a review in your caption. You can create the post and then take a screenshot before the last step if you don't want to post it in your feed.
A good review should include the following:
a short summary
main topics
your opinion: did you like or dislike the book and why
no major spoilers
optional: How would you rate the book? (e.g.: 4/5 or 7/10)