How do you return new Observable(function(observer) { ... with RxJS v5?

I am trying to replace all the promises that my functions return with Observables. From this post, I learned that I should no longer use "new Observable" Observable.forkJoin and array argument

What is the RxJS v5 syntax to achieve this as such to achieve asynchronous waiting?

thirdFunction() {

    let _self = this;

    return new Observable(function(observer) {

        ...

        observer.next( responseargs );
        observer.complete();
    });
}

Thank you greatly for help you can offer.