You will have a rough idea of what you want to do, but will need to 'tweak' your idea by trying things out practically before you write a detailed plan.
You therefore sketch out a rough experiemental procedure and test out the best way to do it in a preliminary session.
You always need to check out the safety issues with a teacher or laboratory technician before you start 'experimenting' - they have more experience at spotting potential dangers than you do!!
You may want to:
What you find out from your preliminary readings will influence your final design of your experiment - so it is an important step.
Remember to say in your report if you found out a better way to do it from preliminary work.... and how you checked your ideas were sound before proceeding.