Often you can just plunge ahead, note that it needs to be there and add it later, on the edit.

This wasn’t one of those.

It took me two WEEKS, but I finally got it.

Very small scene. Most people will never figure why it could give anybody trouble—but brother! Such a pain!

As a rule, the hardest scenes to write are the little ones that ‘direct’ a mood or opinion. This is why most people don’t understand the writerly rule “show, don’t tell.” What it really means is—take the reader on that mental trip: don’t just narrate it…TAKE him there.