How to straighten spine?

A beginner question: how to straighten the spine? I heard several approaches:

  1. Tuck chins in; lift crown gently; adjust it until the neck portion feel good;

  2. Relax the lumbar and cervical vertebrae into their curves; then the sacrum and thoracic vertebrae will naturally set into their curves; tuck chin; lift crown;

  3. Tuck belly in gently; sacrum will drop a bit; push up along the lower curve of thoracic vertebrae, it will expand the lower rib outward; chin in; lift crown.

When I started Yoga a few months ago, straightening spine seems straightforward. But now it gets complicated again.

Namaste

In all poses work to maintain the natural four curves ? lumbar and cervical forward; thoracic and sacral posterior. Always focus on lengthening the spine this will straighten it if there are lateral curves (minor varieties of scoliosis). I do not recommend tucking tailbone forward to flatten the lumbar and abdominal as this inevitably leads to sacroiliac troubles.