Launch the appropriate Electrum/Electron Wallet application on your desktop.
Select the addresses tab.
Right click on the address that you wish to claim and select the "Sign/verify message" option.
Enter the message you generated from the Cereneum website into the area labeled "Message", then press the button labeled "Sign".
This will generate a signed message under the area labeled "Signature".
Continue from bullet point 8 on the Cereneum Claim page.