How to use Arbre gem in rails?

I want to try the Arbre gem for rails. There is an example on this page: https://github.com/activeadmin/arbre/blob/master/README.md

Where must I paste the next code?

html = Arbre::Context.new do
  h2 "Why is Arbre awesome?"

  ul do
    li "The DOM is implemented in ruby"
    li "You can create object oriented views"
    li "Templates suck"
  end
end

I want to try the code above, but I don't know where I must paste it. Which file? Which method? I pasted the code into my controller, but it doesn't work.

In the example you pasted, the html variable now holds the html for your page.

You can render it in the controller like this:

app/controllers/whatever_controller.rb

def show
  html = Arbre::Context.new do
    h2 "Why is Arbre awesome?"

    ul do
      li "The DOM is implemented in ruby"
      li "You can create object oriented views"
      li "Templates suck"
    end
  end
  render html: html.to_s
end

Also, it isn't well documented on the Github page, but digging through the source, it appears you can also use abre templates in place of regular erb views like so (notice the .arb file extension):

app/views/whatever/show.html.arb

h2 "Why is Arbre awesome?"

ul do
  li "The DOM is implemented in ruby"
  li "You can create object oriented views"
  li "Templates suck"
end

That way, your controller could look like this:

app/controllers/whatever_controller.rb

def show
  # nothing necessary here, by default renders show.html.arb
end