After destroying the tip of three nylon spudgers, I would like to advocate using a METAL spudger, as shown in the picture. Just be careful to minimize scarring of the metal cover. But, the nylon spudger definitely is not strong enough to release the chassis, especially since these phones tend to be pretty old and set in their ways at this point in time. With a metal spudger, I was able to separate the back from the chassis in just a few minutes, after spending over 30 minutes failing with the nylon spudgers.
These instructions were really fantastic. SO easy to follow, and I'm especially glad that there was a video which reinforced the written instructions. Thank you!