For RPM Fusion (free repository):
Get the release rpm:
$ wget http://download1.rpmfusion.org/free/fedora/\
rpmfusion-free-release-stable.noarch.rpm
Check the archive's integrity via:
$ rpm --checksig rpmfusion-free-release-stable.noarch.rpm
Which should fail with:
[..] MISSING KEYS: GPG#KEY_ID [..]
Add key to your gpg keyring for checking:
$ gpg --recv-keys KEY_ID
Compare the fingerprint with the published information on the RPM Fusion key site and possibly check the web of trust:
$ gpg --fingerprint KEY_ID
If successful make the key known to rpm:
$ gpg --export -a KEY_ID > rpmfusion.asc
# rpm --import rpmfusion.asc
Check the integrity of the package for real:
$ rpm --checksig rpmfusion-free-release-stable.noarch.rpm
If it is ok install it:
# yum localinstall rpmfusion-free-release-stable.noarch.rpm
This will create config files under /etc/yum.repos.d/ and key files under /etc/pki/rpm-gpg.
Note that the # means that you have to execute those commands as root.
Also the current rpmfusion-free-release-stable.noarch.rpm is signed with the RPM Fusion Fedora 15 key but it is the right setup package for Fedora 17 as well.
For the nonfree RPM Fusion repository you have to wget the analogous setup rpm as well.
For livna.org you have to:
$ wget http://rpm.livna.org/livna-release.rpm
The other steps are analogous.
Fingerprints
As the time of writing the following keys were used:
http://download1.rpmfusion.org/free/fedora/\
rpmfusion-free-release-stable.noarch.rpm
KEY_ID: 00a4d52b
key fingerprint: A828 D083 D794 92E9 C52B C305 ADC5 70DE 00A4 D52B
http://rpm.livna.org/livna-release.rpm
KEY_ID: a109b1ec
key fingerprint: 037B 5D9B E1B6 B673 2A23 13B5 7129 5441 A109 B1E