Go to the Tor Browser Bundle download page. Select the correct download, and download! Below the nice, purple button should be a link that says sig. Don't left click but instead right click it. Then select Save Link As in the context menu. (In Firefox, I don't know in the rest, sorry. In Safari I believe you Alt click the link.)
Once downloaded verify as normal. From Tor Project Docs (It contains instructions for Mac and Linux as well.)
You need to have GnuPG installed before you can verify signatures.
Download it from http://gpg4win.org/download.html.
Once it's installed, use GnuPG to import the key that signed your
package. Since GnuPG for Windows is a command-line tool, you will need
to use cmd.exe. Unless you edit your PATH environment variable, you
will need to tell Windows the full path to the GnuPG program. If you
installed GnuPG with the default values, the path should be something
like this: C:\Program Files\Gnu\GnuPg\gpg.exe.
Erinn Clark signs the Tor Browser Bundles. Import her key
(0x416F061063FEE659) by starting cmd.exe and typing:
"C:\Program Files\Gnu\GnuPg\gpg.exe" --keyserver
x-hkp://pool.sks-keyservers.net --recv-keys 0x416F061063FEE659
After importing the key, you can verify that the fingerprint is
"C:\Program Files\Gnu\GnuPg\gpg.exe" --fingerprint 0x416F061063FEE659
You should see:
pub 2048R/63FEE659 2003-10-16
Key fingerprint = 8738 A680 B84B 3031 A630 F2DB 416F 0610 63FE E659
uid Erinn Clark <email@example.com>
uid Erinn Clark <firstname.lastname@example.org>
uid Erinn Clark <email@example.com>
sub 2048R/EB399FD7 2003-10-16
To verify the signature of the package you downloaded, you will need
to download the ".asc" file as well. Assuming you downloaded the
package and its signature to your Desktop, run:
"C:\Program Files\Gnu\GnuPg\gpg.exe" --verify
The output should say "Good signature":
gpg: Signature made Wed 31 Aug 2011 06:37:01 PM EDT using RSA key ID 63FEE659
gpg: Good signature from "Erinn Clark <firstname.lastname@example.org>"
gpg: aka "Erinn Clark <email@example.com>"
gpg: aka "Erinn Clark <firstname.lastname@example.org>"
gpg: WARNING: This key is not certified with a trusted signature!
gpg: There is no indication that the signature belongs to the owner.
Primary key fingerprint: 8738 A680 B84B 3031 A630 F2DB 416F 0610 63FE E659
Notice that there is a warning because you haven't assigned a trust
index to this person. This means that GnuPG verified that the key made
that signature, but it's up to you to decide if that key really
belongs to the developer. The best method is to meet the developer in
person and exchange key fingerprints.
Then install as normal by running the
.exe, should be
torbrowser-install-X.Y.Z_LANGUAGE.exe in my case
torbrowser-install-3.6.2_en-US.exe. Tor Project has instructions in greater detail for Windows, Linux, and Mac.