If you are in a hurry==>Profiles

  1. Select button: Profiles
  2. Define region with data points
  3. Select method: cubic
  4. Create new layer: Profiles
  5. Define drawing box
  6. Draw the profile projection
  7. Select the profile projection
  8. (Optional) Select Reference level or Surface
  9. Draw the profile
  10. Other options
  11. Add additional profiles
  12. Select (+), Erase, Export
  13. Close
  14. Export

See a video of the whole process