Preparing a first draft of your poster is different from drafting an essay or report, as you'll need to consider the physical space available for you to use.
Because your poster will need to be edited ruthlessly (to ensure compliance with criteria and guidelines), it is important to allow yourself plenty of time to engage with the process.
Editing
Editing means making changes to the content of the poster with a view to improving your argument, cohesion and coherence (flow), and coverage of the topic. This involves identifying any areas that need to be cut and any areas that need to be added to. Aim to avoid repetition and redundancy (information not relevant to the question). Also make sure that content is logically paragraphed (blocked together) and ordered systematically so that it flows and makes sense.
The process of drafting and editing should be repeated until you are confident that you have presented a clear message (visual and textual), your spelling and grammar has been checked, and you are happy to submit your work for assessment.