How to create delegated admin in Lightsail

Hello.

Create the IAM policy required for Lightsail operations and create an IAM user using that policy.
For example, what kind of operations are you expecting?
The IAM actions required to operate Lightsail are described in the following document, so I think it's a good idea to select the necessary actions from this document.
https://docs.aws.amazon.com/service-authorization/latest/reference/list_amazonlightsail.html