The genetic material in the nucleus of a cell (located on chromosomes) has information for the manufacture of specific proteins.  When a secretory protein such as insulin is synthesized by a cell, the information on how to make this protein is copied onto a molecule called mRNA.  This protein blueprint then goes out into the cytoplasm and becomes associated with  ribosomes on endoplasmic reticulum.  A rough form of the protein is synthesized and transported into the ER channels.  As the ER channel fills with insulin molecules, vesicles filled with the molecules pinch off of the ER and migrate to the golgi.  The golgi prepares the molecules for exit from the cell.  Secretory vesicles filled with the final version of the molecule pinch off from the golgi and migrate to the cell membrane.   The membrane of the vesicle fuses with the cell membrane and the molecules are dumped to the outside of the cell (exocytosis).