how to install Microsoft office on a MacBook Pro 13 (2011)

You will need at minimum, macOS Catalina to support Microsoft's current operating system requirements for Microsoft 365, or the single purchases of Office 2019/2021 for Mac. The oldest MacBook Pro that can install Catalina is a mid-2012 device, or if Retina model, Late-2012.

Microsoft has deactivated the license servers for all of its retired products, which means that any attempt of a new installation and licensing for Office 2011 for Mac on operating systems older than Catalina will fail. A boot into Recovery and then Restore from a Time Machine backup of the same operating system would probably be the only way to reinstall Office 2011 for Mac on an older device without the need for Microsoft license activation.

You would need a version of Microsoft Office that is compatible with such an old computer. I don't believe there are any such old versions available from Microsoft. Check with Microsoft.