I tend to work this way with a tool called Markmap (https://markmap.js.org/repl). I start with one heading and a list then begin moving items and adding new headings/subheadings as the relationships form. For example:

  // Start with:

  # Ideas
  - Item 1
  - Item 2
  - Item 3
  - Item 4
  - Item 5
  - Item 6

  // Move to:
  
  # Ideas

  ## Group 1
  - Item 1
  - Item 2
  - Item 3

  ## Group 2
  - Item 4
  - Item 5
  - Item 6