I think the key word above was modern. I felt able to design a simple CPU when I finished my Computer Architecture course in university. I think I forgot most of it by now ;) There are a few basic concepts to wrap your head around but once you have them a simple CPU is doable. Doing this with TTL or other off the shelf components is mostly minimizing/adapting/optimizing to those components (or using a lot of chips ;) ). I have never looked at discrete component CPU designs, I imagine ROM and RAM chips play a dominant part (e.g. you don't just built RAM with 74x TTL flip-flops).