I originally posted this in the Reddit comment thread for something else. I am reposting here to give it a more permanent home, and some additional explanation.
You can do this on paper, or any computer drawing program which supports snapping to edges, corners, etc. and snapping lines to 30-degree angles.
I use Microsoft Visio for all my drawings, as it’s what I use at work and I am very familiar with it. On the free software side, you could use Inkscape or Dia. If you were really serious about precision, you could also step up to using a real CAD package like qCAD.
Visual communication and computer-aided drafting are very useful skills in all walks of life. For an example, watch Adam Savage, of Mythbusters fame, laser-cutting a plastic gear from 1⁄4” acrylic sheet, using a design drawn up with Rhino (a CAD package like qCAD or AutoCAD.)
Click for full size PDF. How to draw voxels in isometric view.