If you are in a hurry==>Volume calculation

  1. Select button: Volume
  2. Define region holding data points
  3. Select method: Cubic
  4. Create new layer: Volume
  5. Define integration region
  6. (Optiona) Specify layer "Project" as a Reference Surface
  7. (Optional) Request Cut & Fill calculations
  8. (Optional) Modify default values
  9. Select: Advanced or Standard volume calculation
  10. Close
  11. Export

See a video of the whole process