npm console missing script: start not working why?

Question

Node.js

I understand that the matter is in package.json, but startI don’t understand where to write it correctly.

{
  "name": "project_name",
  "version": "1.0.0",
  "description": "project_description",
  "author": "author_name",
  "devDependencies": {
    "gulp": "^3.9.1",
    "gulp-uncss": "^1.0.0"
  },
  "dependencies": {
    "gulp-sass": "^4.0.2"
  }
}