Jan has an awesome tutorial for photoshop Elements….
I started by choosing a dark gray colour to work with. The ink used for postmarks in the old days when letters were hand-canceled (I worked for the post office back then) was more of a blue-black than a true black, and it often looked more gray than anything else, as the ink pad aged.
I opened a 12×12 canvas on my workspace so I could have lots of room to manoeuver. Then I pulled out a large circle using the Elliptical Marquee tool with the Tool Option on Fixed Ratio 1:1.
Then I applied a 20 pixel stroke to the inside of my circle.
I know you want to see the whole tutorial, so visit her blog and read the rest