Lewis structures use Local Bond Theory and Valence Shell Electron Pair Repulsion Theory to produce simple initial guesses at the 3D structure of molecules and ions.
Local Bond Theory says that each bond is made of a pair of electrons located between the two nuclei that are being bonded.
(This is disproven by UV Photoelectron Spectroscopy, but that is a topic for another day. We keep using Local Bond Theory because it is very easy to use and it powerfully provides good initial structural guesses.)
Here is a good source for learning the basics of Lewis Structure creation.
This is a part of a series of pages that serve as a static set of URLs to be used in the references section of my book Spectroscopy Theory in One Dimension through AIP Press.