From the Emergency Child Birth online module:
Preparation for birth (after you have determined it is imminent):
1. Assist patient to a firm, flat surface
2. Patient supine* hips and knees flexed and abducted, perineum visible
3. Provide warmth, ensure adequate lighting
4. Place a plastic bag or sheet, sterile drape under buttocks
5. Wash hands, use sterile gloves

*Suggested optimal positioning. Cannot force mom into this position. If she is too uncomfortable, let her guide what the best position is for her.