Hopefully you haven't completely given up or have figured it out, but here's a pretty basic guide.

1. Get the protobuffer keycard

2. Activate the soulbox on one of the protobuffer parts

3. Go talk to the giant pumpkin by fountain

4. Find 5 larger pumpkins around the map and do the soul boxes for them after interacting with them (you have to knife them once you finish filling the soul box)

5. Go back to fountain and talk to giant Pumpkin

6. Shoot all 5 pumpkins in the pentagon (by front of fountain by Elevator) and then interact with the circle by standing in the middle

7. Pumpkins will occasionally fall back to the ground, shoot them as fast as possible keeping the soul growing

8. Once ritual is done, head back to the giant pumpkin and talk to him

9. Take your seasonal reward