npm install --save @ng-bootstrap/ng-bootstrap

app-module.ts

import {NgbModule} from '@ng-bootstrap/ng-bootstrap';

@NgModule({
    imports: [NgbModule.forRoot(), ...],

index.html

< !doctype html >
< html lang="es" >
< head >
  < meta charset="utf-8" >
  < title >Bootstrap< /title >
  < base href="/" >

  < meta name="viewport" content="width=device-width, initial-scale=1" >
  < link rel="icon" type="image/x-icon" href="favicon.ico" >
  < link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/bootstrap/4.0.0/css/bootstrap.min.css" integrity="sha384-Gn5384xqQ1aoWXA+058RXPxPg6fy4IWvTNh0E263XmFcJlSAwiGgFAW/dAiS6JXm" crossorigin="anonymous" >

< /head >
< body >
  < div class="container" >

    < app-root >< /app-root >

  < /div >
< /body >
< /html >