If you are planting a large number of seedlings, consider using a tree planting shovel rather than a generic spade. A tree planting shovel is specially designed to make a slit in the ground that will help you plant seedlings more efficiently.
See the graphic below from the Agroforestry and Woodlot Extension Society which demonstrates the proper planting technique:

Digging Your Hole
When digging a hole for your seedling, it should be as deep as the roots so that the roots can grow straight. The hole should be about twice the width of the root ball.
Preventing Injury
Because planting seedlings is a repetitive task, there is a risk for strain injuries if proper care is not taken. Consider these tips to prevent injury:
- Wear the appropriate Personal Protective Equipment (PPE).
- Work boots, gloves, and a sun hat are recommended.
- Stay hydrated.
- Take frequent breaks.
- Select a shovel with a length that is suitable for your height to maintain a healthy posture.