Napkins. The solution you're looking for is napkins. But, it's true, they're hard to write on, so we'll compromise.

Take a pad of paper and a good pen (hint: Pilot G2) and go to a place that is entirely free of computers. Coffee is optional.

Start sketching. Your goal is a set of notes, an outline, a diagram or two, and/or some paper-based UI wireframes that describe your project. Imagine that you're trying to invite a recent comp sci grad to work on your project with you: What might you sketch? Draw that.

You can use emacs if you want, but on no account should you allow yourself to choose a font for your spec. If you find yourself reaching for the Fonts menu, or wondering whether your spec should have a standardized header and footer, you have stopped planning and started procrastinating.

You need to do some planning, because you don't want to waste time implementing stuff that doesn't even work on paper. But you don't necessarily need a capital-D Document, or even a real presentation. Once the napkin sketch of your finished product is complete, to your own satisfaction, it's time to build the prototype and observe all your mistakes. ;)