Step 1
Open a document at 25x25 pixels. Zoom in about 600% so you can see the pixels. Now create the envelope frame. Remember to leave out the pixels on the four outer most corners. doing so creates an illusion of rounded corners.
Step 2
Create a new layer and draw the seal. it's basically 3px 45 degree angle and then a straight line and then another 3px 45 degree line. Set this layer to 50% opacity.
Step 3
Make a new layer and make a selection that fills up the inside of the envelope.
Step 4
Fill this selection with any color and double click it to go to its Layer Style. Apply a Gradient Overlay. Choose colors that are light. I chose light orange and white. The light orange is on bottom and the white is on top.
Final Result:
