This is a nice essay introduction depending on the type of spec you want to write: https://www.joelonsoftware.com/2000/10/02/painless-functional-specifications

This is a nice essay introduction depending on the type of spec you want to write: https://www.joelonsoftware.com/2000/10/02/painless-functional-specifications-part-1-why-bother/ https://www.joelonsoftware.com/2000/10/02/painless-functiona...