How does silicon dioxide become a covalent bond. (explain)

Silicon dioxide is SiO2.  Silicon has 4 valence electrons, while each oxygen has 6 valence electrons.  This can be shown as 
     **             *                   *
**  O *       *  Si   *        *   O  **
      *               *                 **
At points where there is one valence electron, represented by a lone *, the electrons will be 'shared' between the atoms.  This will make silicon dioxide appear as
   **           * *
**  O *--*  Si   *  *   O  **
      * *                 **
, as the lines with no arrows indicate that each electron moves between the atoms, and does not stay with one forever.