It's very simple. Of course, it takes practice.
5 seconds sound clip from the Miracle on 34th Street (1947) movie soundboard.
You can hear this line at 00:39:38 in the Blu-ray version of the movie.
Quote context
[...]
- Well, that can be caused by other things too.
- How would you like to have a ship all to yourself that makes daily trips to China and Australia?
- It's very simple. Of course, it takes practice.
- Now, the first thing you've got to learn is how to pretend.
- And the next time Homer says, What kind of animal are you? You tell him you're a monkey.
- But I don't know how to be a monkey.
- Sure you do. Here. I'll show you.
- Now, just bend your body over a little like that. Let your arms hang loose. See?
- Now put your right hand up here, under here. That's right. Now scratch yourself. See? That's right.
[...]