Here's how you can sign a message cryptographically. This is needed for art contest winners, but is useful to know in general too.
- Go here.
- Allow access to the Polkadot JS Extension. (Don't have it? Install it here)
- Pick your account and enter the message into "Sign the following data"
- Click "Sign Message"
- Enter your wallet's password and confirm
- Copy the resulting signature by clicking the "Copy" button:
Success, you signed a message.