You register it in `@NgModule` declarations:
@NgModule({ imports: [ BrowserModule ], declarations: [ App , MyChildComponent ], bootstrap: [ App ] })
and then You just put it in the Parent's Template HTML as : `<my-child></my-child>`
That's it.

You *don't put* a component in `directives`
You *register* it in `@NgModule ` declarations:
@NgModule({ imports: [ BrowserModule ], declarations: [ App , MyChildComponent ], bootstrap: [ App ] })
and then You just put it in the Parent's Template HTML as : `<my-child></my-child>`
That's it.