Open a new document in Photoshop with a size of 350 x 300 pixels. Add a new layer by clicking on the Create a New Layer icon in the layers palette. With this layer active (a in front of it in the layers palette), add a Layer Mask by clicking on the Add a Layer Mask icon in the layers palette. This will make the layer mask automatically active. We're going to use this mask in step #3 to add to make the stamp's edges.

Click on the brush icon in the Tool Bar to activate the Brush Tool or press the letter B on your keyboard.
Select the Brushes Palette by using one of the following 3 methods

• Pressing F5 on your keyboard
  
• Selecting the Brushes Palette in the Options Bar
  
• In the menu: Window / Brushes
  

In the Brushes Palette select Brush Tip Shape (A) and select a Diameter for the brush of 17 px (B), a Hardness of 100% and a Spacing of 140 (C). Make sure that all the options underneath Brush Tip Shape (like Shape Dynamics, Scattering, Texture, etc) are not selected.

Press the letter D on your keyboard, which makes the foreground color black and the background color white:
Now press X on your keyboard to switch fore- and background color:
The foreground color should be black now.

Before we're going to draw the dots we need to mark the size of the stamp. Go to the Tool Bar and select the Rectangle Tool or press the letter U on your keyboard. Now make sure that the option Paths is selected in the Options Bar:

Now draw the initial shape of the stamp with the rectangle tool, until you have something like this:

 

Before you proceed with the next step; make sure that the Layer Mask is still active (notice the in front of the layer and the double border around the mask), if not, click on it.

The next step is to activate the Paths palette by clicking on the Paths tab (A), opening the fly-out menu by clicking on the little black triangle (B) and selecting Stroke Path... (C).

In the window that pops up, select Brush and click OK.

 

A quicker way to do the same is by clicking on the Stroke Path with Brush iconin the Paths palette:

Tip: hold down the Alt key (option key on the Mac) before you click on the Stroke Path with Brush iconto be able to select the tool you want to use for the stroke like we saw in an earlier screenshot. By default Brush is selected. Use the method that your prefer. For beginners it might be easier to remember the fly-out menu than all these dozens of icons and that's why I'm showing both approaches.

Ok, so we have stroked the path, but you might wonder why you're not seeing anything change in the paths palette. That is because the path palette only shows vector based shapes and not pixels. Our stroked path is based on a brush, so the stroke was added to the pixel based mask (which was active during the stroke). You'll see soon enough.

Click on the Load path as a selection iconin the Paths palette:

This will convert the path to a selection.

Now click on the Layers tab to activate the layers palette and disable the mask by clicking on the layer's thumbnail with the checkerboard pattern (you'll notice aappear in front of it and a double border around the thumbnail).
Notice that our mask now shows the result of stroking the path:

 

Set the foreground color...

... by clicking on the foreground color in the options bar.

In the Color Picker window that pops up, select your preferred color and click OK.
Select the Paint Bucket Tool in the Tool Bar and fill the selection:

 

You can finish it off by adding some noise to the paper, roughening the edges, adding some color stains and an image, it all depends on how you want it to look.

Have fun!

For more help with Photoshop check out oursection.