Was it just to list out all the card names and numbers and then take SHA256 of the list?
That is the easiest and most direct. It is slightly less than 256 bits of entropy (225 bits for 52 card deck, 237 for 54 card deck).
Or was there some way of assigning bit values to the cards? I left it out of my suggestion, because I couldn't remember how to do it.
You probably could but it would be tedious. I never came up with any such system. At some point one needs to trust something. Still if one absolutely needed to generate the private key by hand only then the simplest (although not the most efficient) would be to just look at the suits but it would require at least 3 (4 is probably better) passes through the deck.