How to write a professional project specification about a web application ?

Good morning ,

Total beginner here , we have a school project about building a Web application (using spirng boot and Angular this application is about animal adoption ) that we will deployed in our private cloud which already done (using OpenStack)

So Our teacher want us to provide project specification for the website application .

Can you please give me some good resources to flow in order to write a clear and good project specification (I find a lot of example in the internet but it seems like a no expert showing you how to write )

Thanks in advance