Examiners like to see plans because they know your essay will be better structured/thought out than the majority of what they're reading. You needn't write anything too complicated (after all the timing is tight enough!), but if you think it will help you, go ahead. As for how to clarify that it is your plan and not your essay, putting it in brackets would be a good idea - maybe even simply using the header 'plan' would be good enough. All past scripts I have seen have the plans crossed out. Whether that be the examiner or the candidate's doing, I'm unsure. A line through it wouldn't hurt though? As long as however you separate your plan and your essay doesn't obscure what's written in your plan, you should be fine.