Thanks, @antonin for the comments and questions.
One things that is perhaps good to clarify is that this internship is just the first step in a long process. The long process is getting designers more sustainably involved in OpenRefine community, but it is part of a much larger problem in the entire software dev industry and OSS dev more specifically - a problem that puts design down as nonessential decoration and prioritizes dev skills, even at the cost of building products that do not adequately meet user needs.
There is an entire design community (https://discourse.opensourcedesign.net/) dedicated to exploring this problem and providing support to new designers entering the field, but there are no silver bullets and a systemic problem like this cannot be solved by a single project.
Another important point to clarify is that the internship idea was inspired by my own experience working on a paid project for OpenRefine and realizing I needed to spend a long time onboarding myself to get a basic understanding of established UI / UX patterns in OpenRefine before being able to meaningfully contribute to the paid project. What I really wanted to contribute in the end of the paid project was proper design documentation and component system, but there wasn't the time/funding for it. So the internship is for me an opportunity to finish this endeavour with help from another person who will thankfully get paid for the work and hopefully also learn something in the process.
So to get back to your question:
I think in an ideal world, we'll have a tiered system of onboarding that welcomes contributions of different scale and skill-level. However, to start with, we should not be overly ambitious and target the most straightforward 'designer' audience - that is professional designers who are needed for paid projects. I think it's unrealistic to expect these people will be users of OpenRefine, so the onboarding should be thorough. For the time being we're focusing on using Figma as a design system tool, which is widely popular among designers, easy to learn / use (due to fact it has far less options than older, more complex products like Adobe or Sketch), and crucially - it is also free, so no financial barriers to participation.
Ideally the documentation will have an easily accessible and highly visible component on the OpenRefine site itself, alongside the documentation for developers / users. Thus it will be possible also for existing OpenRefine contributors (volunteers, students, developers, etc) who may not be formally trained designers themselves to get helpful tips if their contribution also includes a design element.
I am suggesting we take a practical & achievable approach basically, and then iterate in the future and see how this can be developed further after the workflows and documents we produce this summer are tested with some real paid projects / internships in the coming months / years.