How do you make use of it? Is it basically logarithms of values? So add them together to multiply them? I don't know if I quite understand how the units would work.

As an example, "how long would it take for everyone in a large high school to microwave their meals if they all did it sequentially?".

People in a large high school is *4* (let's say about 3000 people).

Microwaving a meal is *2* (let's say 5 minutes).

So by Napkin Math we get 4+2=*6* which is a *large metropolitan area*.

Oops. I mean 11.6 days.

And with normal multiplication, 3000 * 5 = 10.4 days, so 11.6 days is a pretty good estimate.

So I think the answer is indeed "add the numbers together to multiply them", and be careful that you understand what units you're using.