From memory:

Click on the account that you want to use. Then near the top right you will see a sign and verify button. Click it. It will bring up 2 forms that you can fill out, sign and verify. The first field will be the message. Enter the message that you want to sign in the message field. The second field will be the address. begin typing the address that you want to use. it will detect the address you are entering and provide an option to fill out the rest of the address for you. Click sign. Now look at your trezor. It will ask you to confirm that you want to sign the message. Confirm that you do. Now enter your pin. Look back at the form page in your browser. you will see that the formerly empty signature box will now contain a series of characters. That is your digital signature.