Sure, here's a step-by-step guide I once compiled for this:


A. To sign a message:

1. Hold SHIFT down while launching Electrum
2. (Only if the address is external and must be imported)
    Import your address by typing:
    e -o import 1YourBitcoinAddress:5YourPrivateKey
3. In the virtual console that has appeared, type:
    e signmessage 1YourBitcoinAddress "MessageLine1\nMessageLine2\nMessageLine3"
  
    This should output the base64-encoded cryptographic signature.    


B. To verify a signed message:

1. Hold SHIFT down while launching Electrum
2. In the virtual console that has appeared, type:
    e verifymessage 1YourBitcoinAddress "TheSignature" "MessageLine1\nMessageLine2\nMessageLine3"    
     (Be sure to include the quotes.)

   If successful, this must return True.  


Note that all of the above can be performed on a non-networked computer, if preferred.

PS: hopefully in a future release, these commands will be accessible from the graphical interface. For now, they are only available through the advanced (text-based) console.