How to connect input with class in a simple way

Cleanliness is somewhat subjective, but this might be something you're looking for.

class Intro

  def self.display
    input = nil
    until (1..5).include? input
      puts "Welcome player! Choose one of the following!"
      puts "1. Create a new flash card?"
      puts "2. View all flash cards?"
      puts "3. Edit a flashcard?"
      puts "4. Delete a flashcard?"
      puts "5. View score and answers?"
      input = gets.chomp.to_i
      puts "Invalid option." unless (1..5).include? input
    end
    self.select(input)
  end

  def self.select(number)
    @number = number
    puts "#{@number} selected!"
    puts "#{@back}" # @back is uninitialized
  end
end

Intro.display

Edit:

If you want to be able to have multiple answers then one way to go about it would be to create instances of your Intro class each with a instance variable containing the answer.

So, instead of calling display on the class itself you would call it on an instance of the class.

The class Intro would have an initialize function that is automatically called whenever a new instance of the Intro class is instantiated.

class Intro

  def initialize(answer)
    @answer = answer
  end

  #[...]

This would mean that by calling

new_game = Intro.new(7)

you are creating an instance of the Intro class that has its instance variable @answer set to seven and you are storing that instance in the variable new_game. And from there you can call instance methods on new_game.

Your question is very broad because there are many ways to write it. As a first pass I'd do:

class Intro
  CHOICES = {
    1 => 'Create a new flash card',
    2 => 'View all flash cards',
    3 => 'Edit a flashcard',
    4 => 'Delete a flashcard',
    5 => 'View score and answers',
  }

  def self.display
    loop do
      puts "Welcome player! Choose one of the following!"
      CHOICES.each do |h|
        puts '%d. %s?' % h
      end

      input = gets.to_i

      return input if CHOICES[input]
      puts 'Invalid option.' 
    end
  end

  def self.select number
    puts "#{number} selected!"
  end
end

choice = Intro.display
Intro.select(choice)

Your choice of gets.chomp.to_i is better written as gets.to_i.

(1..5).include? input is fine but forces you to edit your code if you add or remove options. Instead, compute the maximum value based on the list of choices, so you only have to add or delete them and everything adjusts.

self.select input probably shouldn't be a separate method. It probably shouldn't be called by display either, but that's mostly a programmer-choice.

Using a class for such a simple piece of code is overkill. It could be simplified to:

CHOICES = {
  1 => 'Create a new flash card',
  2 => 'View all flash cards',
  3 => 'Edit a flashcard',
  4 => 'Delete a flashcard',
  5 => 'View score and answers',
}

def display
  loop do
    puts "Welcome player! Choose one of the following!"
    CHOICES.each do |h|
      puts '%d. %s?' % h
    end

    input = gets.to_i

    return input if CHOICES[input]
    puts 'Invalid option.' 
  end
end

def select number
  puts "#{number} selected!"
end

choice = display
select(choice)

Using a class to encapsulate the methods, without using it to have instances of the class gets old very quickly. Ruby allows us to write code using classes, or without. Think of what is concise and simple to understand as you write. Sometimes we start simply and have to back up a little and take another run at it because we decide using classes makes more sense. I usually let the complexity of the program determine that.

Using a series of if/elsif/then statements is sometimes useful. We often can use case/when to clean the code up even more than using if chains:

CHOICES = [
  'Create a new flash card',
  'View all flash cards',
  'Edit a flashcard',
  'Delete a flashcard',
  'View score and answers',
]

def display
  loop do
    puts "Welcome player! Choose one of the following!"
    CHOICES.each.with_index(1) do |s, i|
      puts '%d. %s?' % [i, s]
    end

    input = gets.to_i

    case input
    when 1 .. CHOICES.size
      return input
    else
      puts 'Invalid option.' 
    end
  end
end

Replace the previous methods with these and experiment.

...how ('%d. %s?' % [i, s]) [works]

Notice that '%d. %s?' is a String. That'd make % a method of String.

See String#%