how do I install rosetta 2 on my m1 MacBook Air?

A few articles may be helpful for basics.

For use w/ intel-based applications on Silicon.

Rosetta2 only works with later Apple Silicon.

[And developers should upgrade their apps so

Rosetta would not be needed sooner or later.]

• If you need to install Rosetta2 on your Mac - Apple Support

https://support.apple.com/en-us/HT211861

• About the Rosetta Translation Environment | Apple Developer Documentation

https://developer.apple.com/documentation/apple-silicon/about-the-rosetta-translation-environment

You don’t. If and when you open a program that requires Rosetta 2 to operate Rosetta 2 will be downloaded and installed automatically. No need to download it and install it manually.