Preparation

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 1000x300 pixels

In Matrix 

1. sign in
2. select 'User Settings' from the menu
   
   
3. enter your 
   1. new login password (1), 
   2. upload the image (2), 
   3. password for e-signatures (3), (depending on your configuration this might not be visible) 
   4. add you current login password (4)

Look at the video (direct link here) to see how to add the scanned signature to your profile: