How difficult is it to replace rubber seal between back window and engine lid? Is there a step by step process described somewhere? Thanks in advance.

pretty sure you take off the old one and push on a new one

Its as complicated as you want to make it.
Some remove the lid. You dont have to.

So, it's press fit into a groove or something like that? Just pull it out and new one presses in? Maybe it is that simple but I thought it would involve taking the engine lid off. Maybe I'm just stupid.

The OE one is/has a serrated metal "spine" that can often remain & come apart when removing. Other than that...
...what they said.