Casts for scoliosis are now rarely used  because modern instrumentation. The technique is as follows

• Place the patient on a Risser table and apply a stockinette to extend from over the head to the knees.
• Position the removable crossbar at the level of the upper portion of the shoulders. Use felt to pad the canvas strap on which the patient is resting.
• Pass muslin straps around the waist over the stockinette and tie them at the level of the greater trochanter on the opposite side. Then pass the straps through the windlass at the end of the table and apply a slight amount of traction.
• Pad the iliac crest with felt.
• Use extra-strong, resin-reinforced plaster and extend the cast to the sternum anteriorly and the upper portion of the back posteriorly.
• Mold the cast well around the pelvis and iliac crest.
•  As the cast dries, trim it at the level of the pubic symphysis anteriorly, extending proximally to about the level of the anterior superior iliac spine to allow 100 degrees of hip flexion. Posteriorly, trim low over the buttocks at the level of the greater trochanters. Then trim proximally to relieve pressure over the sacral prominence.
•  Remove an abdominal window to free the upper portion of the abdomen, the lower costal margin, and the xiphoid process.

Canale & Beaty: Campbell's Operative Orthopaedics, 11th ed.