Essay plans are normally based on past questions. The same sort of questions often re-appear, so you can prepare essay plans and adapt usually. Some people also like to categories the past questions into buckets and then make essay plans off those broader categories (at EconTools we categories them into broad 'themes' so you can see which appear more, and there's definitely one's that appear way more!). A good essay plan will have your thesis, then maybe your 4 main points that support that thesis (can change depending on how you structure essays). Under each main point you should have maybe 2-3 sub-arguments that support that point, including the theory/concepts that support those sub-arguments and the evidence/stats to support the sub-arguments.
You could definitely do it for prelim if your exams are done in a similar way
Reach out directly if you need any more help.
