How to document Data Mapping on small projects?

Question

JSON

Hello! Tell me an open-source tool that could describe ETL processes for small projects.

I'll give you a case. For example, I want to integrate our backend with 10-15 third party web services. For each web service, I have to write a set of rules, match the fields for converting our data for an external API.

And I wanted to know if there is a tool on the market for my purposes?

PS
I looked towards Data Mapping-emterprise solutions, but I understand that they are too expensive and do not fit the scale of my tasks