http://vector.tutsplus.com/tutorials/illustration/create-a-detailed-envelope-illustration-in-adobe-illustrator/

In the following steps you will learn how to create a detailed envelope illustration in Adobe Illustrator. For starters you will learn how to setup a grid and how to create a pattern and a simple graphic style. Next, using that pattern along with some basic tools and vector shape building techniques you will learn how to create the essential pieces of the envelope. Moving on, using basic effects and taking full advantage of the Appearance panel you will learn how to add shading, depth and highlights for the overall illustration. Finally, using mainly the Appearance panel and some basic masking techniques you will learn how to create a stamp and two subtle seals.