First, to quote JohnZ, "Do you have a 68 Chassis Service Manual?"
It has a clear diagram and 9 steps (9 in reverse to put it all back together).
I've not done it for myself (My Camaro has AC and I take that stuff to a shop!), but I did help when a friend replaced the core in his 68 Chevelle. (OK, I read the steps, drilled the stud, and ran to the store for more coolant!)
It is pretty much the same in all Chevy vehicles. The manual does not show any special tools, but a Camaro does reguire that you drill out an air distributor stud. (Really not that hard: you drill it out and replace it with a new screw and pall nut.) Beyond that there is just the usual disconnecting of hoses (they suggest plugs for the core tubes) and taking off various connection nuts.
What you are actually doing is pulling the air distributor duct to get at the core and replace it. My friend was very careful (and lucky) and did not have to replace the seal. But a new seal wouldn't be a bad idea.
Hard? Good question... I'm not all that mechanical. But my friend is a good weekend mechanic and has a pretty good shop and plenty of confidence. The manual tells you everything --even to the point of telling you, "lay the distributor duct on the floor".