To stretch the spine completely it helps to understand the ways in which the spine can move. Although different parts of the spine have different degrees of movement, all parts can bend and twist to some degree.
To stretch along the length of the spine evenly it helps if you focus on feeling the individual parts of the spine. So that you know that all parts are being stretched evenly, (according to the amount of movement each part of the spine has) look to create a feeling of even tension throughout the spine.
The basic movements of the different parts of the spine are:
Further movements, due to the segmental nature of the spine, include S-bending the spine. In terms of the major elements of the spine (the skull, ribcage and pelvis) this type of movement can result in a sliding movement of the ribcage relative to the pelvis or of the head relative to the ribcage.
Reversing references it can also mean the ribcage sliding relative to the head or the pelvis sliding relative to the ribcage.
Generally a forward bend of the spine results in the front of the spine shortening. It stretches the back of the spine.
A backward bend of the spine does the opposite, shortens the back while stretching the front of the spine.
A bend to the left shortens the left side of the spine and stretches the right side.
A bend to the right shortens the right side and stretches the left side.
To do a forward bending spine stretch I like to start with dropping the sacrum so that the pelvis tilts rearwards.
This creates a forward bend in the lumbar spine. This can be accentuated by dropping the chest.
Because of the ribs being positioned to the front of the thoracic spine it can be difficult to bend the thoracic spine forwards. However it may be made easier by sliding the ribs relative to each other in such a way that the thoracic spine bends forwards.
The spinal front bend can be completed by tilting and sliding the head forwards so that the chin moves towards the chest.
Because of the weight of the ribcage at the front of the spine, standing spine stretches for the back of the spine can be relaxed. You can focus on letting the ribcage sink down and the head sliding forwards and down.
You could make it active by contracting the abs and intercostals and the muscles at the front of the neck.
To stretch the front of the spine while standing, I like to start with lifting the sacrum so that the pelvis tilts forwards. This creates a backbend in the lumbar spine by way of the spinal erectors contracting. This action can be carried upwards so that the thoracic spine also bends backwards.
The stretch to the front of the spine can be accentuated by lifting the ribs and creating space between adjacent ribs.
To stretch the front of the neck you could additionally tilt the head backwards.
I tend to prefer to keep my neck straight when stretching the front of the lumbar and thoracic spine.
One way that I've found to make neck stretches more effective is to anchor the part of the ribcage I am leaning the head towards.
For twisting, I've found it helps to actively (using abs and intercostals) turn the ribcage partially in the direction that I'm turning the head. Then the muscles that are being used to stretch the neck in a twist have a foundation from which to act.
To stretch the spine in a twist while standing I like to use the legs to anchor the pelvis.
Doing a standing twist I could have students use their legs to keep their pelvis facing the front and then have them use their abs and intercostals to turn their ribcage and stretch the spine in rotation.
Another option is to turn the ribcage and pelvis to the right and then keeping the ribcage in place, turned to the right, turn the pelvis back to the front.
Using an understanding of the thoracolumbar fascia, I sometimes activate opposite side glutes and lats while twisting. Twisting to the right I find it helps to activate the left side gluteus maximus and the right side latissimus dorsai to help get me deeper into a twisting stretch for the spine while at the same time keeping my spine feeling comfortable.
When doing spinal stretches, I'd recommend pairing stretches. Obviously if you twist or side bend to one side then you twist or sidebend to the opposite side. But also, if doing backbending spinal stretches then also do front bending spinal stretches and vice versa.
Depending on mood (and if teaching depending on ability of my students) I may do these stretches standing or sitting or even using a wall. Each has their advantages.
When doing sliding actions, of the ribcage relative to the pelvis or of the head relative to the ribcage, I generally like to do these sitting when working with beginners because it is easier with the pelvis stabilized by the floor. One key point when sliding the ribcage to the side is to differentiate between a sidebending and sliding.
If sliding the ribcage to the right and I have students who are bending to the right instead, I'll instruct them to lift the ribs on the right side (and perhaps even to drop the ribs on the left side.)
Sideways ribcage sliding can be a good exercise for fault finding psoas and quadratus lumborum functionality. These muscles can affect the shoulders as well as the hips and so I often combine ribcage slides with either hip stretches or shoulders stretches (or both.)
And I also like to combine them with spinal twisting and sidebending stretches.
I like to do frontal ribcage sliding for further spinal awareness.
Sliding the ribcage relative to the pelvis may not result in a super spinal stretch but it does build awareness and control that can be used to make other spinal stretches more effective or to improve alignment in thos stretches.
On occasion I'll also use head slides, moving the head sideways or frontwards or backwards relative to the ribcage.
Knowing how the spine can move it can be easier to implement specific spinal stretches while doing yoga poses.
As a yoga teacher, I'm constantly exploring new exercises, new ways of doing yoga poses.
There is no single "right way" of doing a yoga pose. Instead, there are options. And the better you are at "feeling" your body, the better you can get at choosing the right option for your body as it is now.
For any technique, the point of practice is to learn feel it and to control it, so that it can be used without thinking about how to use it.
And that is more or less the approach taken in all of my ebooks and videos. They help you to feel your body and control it so that you can work towards using it effectively in anything that you do.