Laying out the circuitry is only part of the story. There is software (firmware actually) which drives the hardware. The program isn't all that difficult to write, but you have to have the necessary equipment to blow the chip. And trust me on this, you'll be writing the program over and over before you get it bug free.
As for plans, a few years back, there were some plans published in QST for such a device, including the requisite phase detector. I don't remember too much about it, but the truth was, the cost of the parts were darn close to the purchase price of the Turbo Tuner.
While I think it is worthy to investigate making your own, if you don't have the hardware, you're better off buying one, and the Turbo Tuner works fairly well. If you have an Icom 7000, you need to look at the BetterRF unit too.