1. After scanning your signature, open your image. Now you will need to separate the signature from the background/paper. Select the magic wand tool. Set the tolerance to about 50 and uncheck CONTIGUOUS. Click on the white section of the paper then SELECT>INVERT. The whole signature should now be selected. If not, you may need to change the tolerance settings.
2. Optional: Next go to Select>Modify>Contract. Contract by 1 pixel. This just makes the selection slightly smaller and will get rid of any anomalies at the edge of the selection. You can experiment with this, you may not need any contracting or you may need two or more pixels.
3. Next choose Edit>Define Brush Preset and name your brush in the dialog box.
That's it! You've created a custom brush of your signature.
To test out your new brush click on the Paintbrush tool. In the Options Bar click the upside down triangle next to the brush shape and size option. Scroll down to the bottom to your new brush. I suggest that you paint your signature on its own layer. This way you will be ability to move and resize without affecting the image below it.