First you have to open a new document, about 300 x 200, of course bigger if you want to make a giant stamp. Fill it up with any color you like , I like (#e7e7e7).
Second step. Make a new layer( Layer-New Layer) .With your Rectangle Tool draw a white rectangle in the center of the page. To make it white you have to set your foreground color white.
Now we will set the Eraser tool. For this you have to select it and to open the eraser panel. In the options bar you will find this button.
Click it and you will have a new panel opened. Here set everything like I did.
Now with your Rectangle Tool selected, you go up in the options bar , under "Image" and "Layer" options and from the 3 rectangles you have there you pick the one in the middle.
Now draw a rectangular over the one you already have . Make sure the corners are the same. It should look like this.
Add a layer mask to your second layer. Select the Path Selection Tool (A) and now right- click in the middle of the rectangular and choose Stroke Path.
Here pick the eraser and click ok. Now you have a stamp.
I added a picture and here is the result. You do this just by drag and drop the image.
