Now that you have created an account, learned the language, and heard examples of what other people have talked about. It is time for YOU to decide what you want to write about and make your first post! The best way to start is by deciding what film you would like to review and watching or rewatching it. Once you have done this start writing! It can be whatever you would like as long as it remains within the community guidelines you must follow on Letterboxd. Once you have written your review, the next course of action is to label whether or not your review has spoilers in it or not! This is very important because many Letterboxd Users will read spoiler-free reviews prior to watching a movie, and while you are allowed to post a review with spoilers without labeling it, it is very frowned upon! Additionally if this is not your first time watching this movie there is an option to select that your review is not a first time watch, this is also not mandatory but it is nice if you decide to revisit an old film! Once you have done that, you're all set to go ahead and post your reviews!
Enjoy the Letterboxd community and expressing your creativity!