Once you have finished verifying the file, you will need to sign the software key with your key in GNU Privacy Assistant

Right-click the key you created in Step 3 and select Sign Keys and use the passprase you used with your private key.

Once signed your key should read Fully Valid under the Validity column.

You can check the validity of your Tails OS image file by importing the file directly into GNU Privacy Assistant and clicking the Check Signatures of Selected File. If the file has not been modifyed then you should get a message indicating the file is valid.