I am about to break a cardinal rule of 'Soto Zen' (not the first time): However, I hereby officially state that I do not think that the 'Lotus' Postures (although ideal if you can) are necessarily the best postures for most beginners ... I suggest the Burmese ...
... and, as a distant fourth, the 'Seiza Bench' (I think many people sit this way because they have not given the other postures a sufficient chance)
... And I view chair sitting as to be done only if truly necessary. We have had some nice chats on our Forum about sitting posture, such as HERE.
We will talk more about this tomorrow, and what to do with the rest of the body from head to toe.
Please see a video by my teacher, Nishijima Gudo Roshi, on how to sit in the Full Lotus posture, look HERE
Press on arrow for 'play'