As a developer, how should I (non technical) lay out my idea for a software project so you know how to build it?

I have an idea for an inventory management system for small businesses in hospitality that I am currently on a pre accellerator program to find a technical co-founder to help me build.

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I know what I want the system to do but I'm really at a loss as to how I describe it. It will basically link up to a point of sale systems database and watch for actions made concerning product items then it will adjust and recalculate based on those actions to give you a real time iventory count. I have a developer who is interested in coming on board but I just dont know how to put my thoughts onto paper. Is there a specific terminology used for this sort of thing. How step by step do I need to go when i dont know how the thing will be built?

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Any help would be greatly appreciated.

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Thanks