1. write it on a piece of paper (rather a small to make the document look nicer) maybe a few times
  2. scan it
  3. crop it (e.g. using windows paint) 
  4. make sure the size does not exceed 800x300 pixels

In Matrix 

  1. sign in
  2. select 'My Profile' from the user menu

  3. enter your 
    1. signature image by selecting and uploading it (1)
    2. add you current login password (2)
    3. Save (3)

Now the image of your signature will be added in your signed documents.