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 correct:

"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 protected]>
uid                  Erinn Clark <[email protected]>
uid                  Erinn Clark <[email protected]>
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 C:\Users\Alice\Desktop\tor-browser-2.3.25-13_en-US.exe.asc C:\Users\Alice\Desktop\tor-browser-2.3.25-13_en-US.exe

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 <[email protected]>"
gpg:                 aka "Erinn Clark <[email protected]>"
gpg:                 aka "Erinn Clark <[email protected]>"
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.