My steps which worked:

gpg --no-default-keyring --keyring ~/tmp/trustedkeys.gpg --fingerprint
gpg --no-default-keyring --keyring ~/tmp/trustedkeys.gpg --import ~/tmp/tails-signing.key
gpg --no-default-keyring --keyring ~/tmp/trustedkeys.gpg --verify tails-amd64-6.4.img.sig

Checksums:

sha256sum tails-amd64-6.4.img tails-amd64-6.4.img.sig  ~/tmp/tails-signing.key
f8c36fad61a7f8c0fce45202369f85499a1c90f1bc7e5e5b320f2de1c3fa4e8d  tails-amd64-6.4.img
bbdd3df622d04f93b40070cd34ba423f7aeb1a7705d65b9da9cc598fc7d8e848  tails-amd64-6.4.img.sig
73c2e478295be78d625220cd6cf7035caeab0a9c48379305ec86e83d43add953  ~/tmp/tails-signing.key