Electrum v 2.9.3
Open Electrum, go to "addresses" then copy the address you want to sign.
Now click on "tools" and then "sign/verify message" from the drop down menu.
Enter your message in the first box (remember to include the date and exact reason you are signing the address) next paste the address you want to sign in the provided field and hit "sign"
You will now have the signature string displayed below the bitcoin address.