Does this conversation sound natural? No, it doesn't. Here's the natural version.

A: Can you tell me how to restart this machine, please?
B: Sure. It's easy. Just press this button.

Clive

Please guess when they talk to each other over the phone. OK, but you didn't say that.

On the phone, you wouldn't usually begin by simply speaking of 'this machine'. 

(Begin by saying things like 'Hi B, this is A. I'm having trouble with the photo-copier.)

A: Can you tell me how to restart it, please?
B: Sure. It's easy. Open the top cover. Do you see the small green button on the back of the cover?

A: Yes, I do.

B: Just press it/that.

Should it be "press the small green button" instead of "press a small green button" because there is only one small green button on the back of the top cover? Yes.

Clive

Hi again,

English grammar books published in Japan explains:
"This is the doll that he made yesterday," suggests that he made only one doll yesterday. Yes.

 It also suggests that the listener is already aware that a doll was made.

You might like to consider these examples.

example 1.

I don't know you make dolls.

You walk into the room and say 'This is a doll that I made yesterday'.

I say 'Oh, how nice'.

example 2.

I  don't know you make dolls.

You walk into the room and say 'This is the doll that I made yesterday'.

I say 'What doll? What are you talking about?'

"This is a doll that he made yesterday," suggests that he made more than one doll yesterday. I'd say it depends on the context.

Nevertheless, can I use "This is a doll that he made yesterday" in the following cases?
1. I do not know how many dolls he made yesterday (he might have made one doll or more than one doll). Yes
2. It does not look like a doll (because he is clumsy and the person I am talking to said, "What's this waste cloth?" In this case, it does not matter how many dolls he made yesterday.) Yes

Clive