Start from simple sleeping on your back...
or on your side.
If you are longhair, use the benefit of your coat and tail.
Remember that you and your box must nearly amalgamate.
Use your imagination: try S-like positions..
or C-like positions...
or even L-like ones..
If you totally trust your humans, relax your back legs to the maximun.
Sometimes O-like positions do not fit the box well, but you can always make an experiment.
If the box is rather small, try sticking your paws&tail out of it...
or stretch just one paw out, like this (back paw)..
or like this (front paw)..
Invite your friend to join you...
and enjoy it.
Your humans may wonder how you can sleep like that..
ignore them and enjoy..
Perhaps they will not even notice you..
Try disguising to avoid unwanted attention..
or you can hide in an absilutely unexpected box..
This position is for experienced yoga cats..
as well as this one..
Even small boxes can do..
practice makes perfect. To get more experience, use as a box any structure you can find..