Open up photoshop and make a new document 500px by
500px.
Select the pen tool
and make a shape similar
to the picture below.
Select the anchor point tool
and add a point the
bottom of the image above.
Now pull the anchor point down while holding shift to
give the flat surface a curved shape. 
Right click on the line and select “make selection”.
With the path selected go to “select” and select
inverse.
Now select the paint bucket tool
and dump
#bf7500 as your color.
Select the pen tool once again and make a square.
Now we make the bottom flap of the envelope.
Right click on the layer with the first flap and
duplicate it.
Use the free transform tool on the new layer and rotate
it around to make the bottom flap of the envelope.
Now move it down to the right position so it looks like
the pic below.
Make sure that the 1st flap is on the top
layer so when we add the shadow it creates the illusion of the paper
overlapping.
Right click on the 1st flap layer and go to
blending options to add the shadow. Apply these settings to the drop shadow.
Select all the layers by clicking on the top layer
while holding shift then click on the first layer. It should look like the pic
at the bottom.
Now press ctrl&E to merge the three layers.
We want the paper to look a little more real so we will
add some texture to it.
Right click on the layer and go to blending options
once more but this time select pattern overlay. Use the settings below to apply
the pattern.
Now add a drop shadow to the envelope and you are
finish. 
