Open Electrum, go to "Addresses" and then copy the address you want to sign. (If your addresses are hidden please go to "View" and select "Show Addresses.")
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) and then paste the address you want to sign in the provided field and click "Sign."
You should now have the signature string displayed below the bitcoin address.