How did Charles Lindbergh use dead reckoning over the Atlantic ocean? Wikipedia and other sources assert that Charles Lindbergh used dead reckoning on his historic flight from Long Island to Paris but—likely being aimed at a broader audience—are short on specific technical details.

With few or no landmarks to use as references, how was Lindbergh able to stay on course while over the Atlantic ocean?
It's actually "ded" reckoning (short for deduced), and it doesn't involve landmarks, it is simply flying a heading for a certain amount of time, that's it.  When you start adding in landmarks and things, that is called "pilotage."

Typically, visual navigation over land is mostly pilotage, with a bit of ded reckoning occasionally.  When you get over the water, like Charles Lindbergh, you do mostly ded reckoning.  When you do have a chance to use a landmark, you can update your location and basically restart your ded reckoning from a known point.  Lindbergh also used the stars to update his position and based new ded reckoning tracks on those positions.