I meant to write this post yesterday, but I had an article deadline breathing down my neck and a post-flu-shot Husband languishing in abject misery on the couch.
In this step you start at the beginning of the outline you made in Step 2 and build the scene sentence by sentence. I literally delete each line of the outline once I've incorporated in into the scene (as always, make sure to save an intact copy of the outline!), so the scene grows down the page as the outline shrinks line by line. It's more satisfying that way.
Some notes in the outlines might require only a few words of description or clue-dropping in the story, while some might generate a few paragraphs of action or introspection. Add attributives and incidental action around the completed dialogue when you come to it.
At this stage, you are not worrying about making it sound good. Think of it as an advanced version of the outline.
Since you are not worrying about making it sound good, and you are not having to stop to think of what comes next, this really shouldn't take more than an hour, unless it is either a very long scene or you are a very distractible person.