connect the external drive to the computer. You should ensure that the external drive is formatted correctly, by launching finder / applications / utilities / disk utilty - then selecting the external drive in the left column of the disk utilty window, then selecting the "Erase tab". This will not erase the drive, but will show you its current format. It needs to be "Mac OS Extended (journaled)"
Quit out of disk utility, and in the finder window, select the externa drive. Go to file / new finder window, and in this new window, go to your pictures folder, and drag the iPhoto Library to the external drives window.
This will prompt that it is copying. Just wait for the copy to finish.
Make sure the drive is formatted Mac OS Extended (Journaled)
1. Quit iPhoto
2. Copy the iPhoto Library from your Pictures Folder to the External Disk.
3. Hold down the option (or alt) key while launching iPhoto. From the resulting menu select 'Choose Library' and navigate to the new location. From that point on this will be the default location of your library.
4. Test the library and when you're sure all is well, trash the one on your internal HD to free up space.
Read teh article yoou posted from??
Moving the iPhoto library is safe and simple - quit iPhoto and drag the iPhoto library intact as a single entity to the external drive - depress the option key and launch iPhoto using the "select library" option to point to the new location on the external drive - fully test it and then trash the old library on the internal drive (test one more time prior to emptying the trash)
And be sure that the External drive is formatted Mac OS extended (journaled) (iPhoto does not work with drives with other formats) and that it is always available prior to launching iPhoto
And backup soon and often - having your iPhoto library on an external drive is not a backup and if you are using Time Machine you need to check and be sure that TM is backing up your external drive