I have a small freelance client whose site I've offered to port to D8 pro-bono as a learning exercise for myself and a friend who's learning Drupal. It'll take a number of months to do, even though architecturally it's a small site, but that's fine. My very first step was to create an initial plan on what steps we'd need to do to to plan the site build. Seeing as this list ended up being completely generic, I figured I'd share it.

A plan for planning a site migration to Drupal 8

• Create site architecture plan
• Create content type plan
• Create vocabularies plan
• Create data migration plan
• Create theming plan
• Identify what contrib work needs to be done
• Identify what custom work needs to be done

And that's it! But it at least gives us a plan on how to start planning our work.

I'll try to post more info as we go.