Answer:
There's not really any specific way.
Step-by-step explanation:
Billie Eilish is a very distinctive singer and there is no real exact way to sound like any particular singer. If you really want to sound like someone the best thing to do is just listen to a particular sentence (or lyric) and repeat it over and over again and each time if your heading in the right track you could sound a bit like her. But there is no exact way to do this.