A Frost diagram is usually applied to all-carbon, monocyclic, pi systems.  It allows one to find the number of molecular orbitals in the molecule's pi system and their energetic locations. To construct a Frost diagram, proceed as follows:

![enter image description here][1]

 - Draw a circle and inscribe the polygon with a vertex located at the bottom of the circle.
 - Energy-wise, the top and bottom of the circle are defined as $\ce{+2\beta}$ and $\ce{-2\beta}$ respectively; the center of the circle is at $\ce{0\beta}$; other points can be interpolated accordingly; the bottom is lower energy than the top of the circle.
 - Wherever a vertex of the polygon touches the circle, that is the location of a molecular orbital.
 - Include an $\ce{\alpha}$ in front of the $\ce{\beta}$ term in order to state the energy of the molecular orbital (MO)

Benzene has one MO energetically located at $\ce{\alpha-2\beta}$.  It also has two degenerate MO's located at $\ce{\alpha-\beta}$.

  [1]: https://i.sstatic.net/Ss3u1.gif