How to convert a Custom Action in Classic to a Smart Action?

Summary:

I have custom actions that call object functions to create records and set certain field values in Classic. I need to replicate these as Smart Actions in Next Gen.

Content (required):

These custom actions allow users to easily click a button to have a new record created, assigned to the appropriate person or team and with all relevant information.

Version (include the version you are using, if applicable):

23B

Code Snippet (add any code snippets that support your topic, if applicable):