If you choose to pair write, you will need to book a meeting, session or workshop. The length of the session depends on how complex the content is.
You may prefer to have several smaller sessions or one may be enough depending on the size of the project and how complex it is.
Before you begin to write, choose a tool for managing the draft. The tool you choose will depend on how you want to work with your partner. For example, Microsoft Word with tracked changes can allow you to have discussions before changes are made and keep a track of them.
It is best to share your screen so you can both work on the same thing.