Rei is manners & Rei is the physical act of bowing. Thus 'upright' bowing, showing respect and humility from a postion of strength is the physical manifestation of manners.
Rei begins and ends with the Tanden.
Awareness is the first step. Awareness of the Tanden is the core of the pracitce. All practice leads with... to... and from the Tanden. With the mind stable and established in the Tanden, we look out... gazing gently... seeing all that is, as it is. In this way we move in the world, with the world... without moving away from the Tanden. Thus we are moving without moving.
The path to the Tanden has always been breathing and relaxing. The path from the Tanden has always been bone and extension... Structure and Vision.
Awareness of the Tanden, through the art of breathing & relaxing, then, is the first step. This is a natural step that happens whenever we do not interfer. Like this, through gravity, with a small tether to the Tanden (like a plumb bob) we drop to the center of the earth. From the center of the earth we stand up; from the sacred tail/root bone we stand up; the spine, nape of he neck and crown of the head reach to the heaven (never leaving the center of the earth, or the Tanden, but merging the earth and sky with the tanden as the center of this universe. From Heaven to earth we hang... suspended...
From this upright posture, anything is possible... everything is possible.