example.rb

class someController < ApplicationController 
  def a
    set_instance_variable
  #How would i access @instance_variable in this method. 
  #I could make it a class variable right? 
    @instance_variable == 'whatever' #=> true
  end 
  
  def b
    set_instance_variable
    @instance_variable == 'whatever' #=> true
  end 

  private
  
  def set_instance_variable
    @instance_variable = 'whatever'
  end
end