How do you document an existing SQL database owned by someone else?

I am working with an existing Microsoft SQL database that does not have documentation. I want to write documentation for myself. However, because I did not create this database myself, I want to avoid editing said database. This means I cannot use Extended Properties or create new Database Diagrams in Microsoft SQL Server Management Studio. What tools and processes are recommended for documenting an existing SQL database owned by someone else? *(Note: this differs from the similarly named question which is about documenting a database you own as it is developed)*