What is the correct order of associations, scopes, validations, etc. in Rails models

There is no such convention. But you can create one for your project and be consistent with it in all the models. This is what i follow.

class Model < ActiveRecord::Base
   #all mixins
   include Something
   extend Something

   #other stuff
   acts_as_taggable
   paginates

   #associations
   has_many :something
   belongs_to :something_else

   #validations
   validate_presence_of :something

   #scopes
   scope :something

   #instance methods
   def instance_method
   end

   #class methods
   def self.method
   end

   #private methods
   private
   def method2
   end
end

Rubocop has it https://github.com/rubocop-hq/rails-style-guide#macro-style-methods

class User < ActiveRecord::Base
  # keep the default scope first (if any)
  default_scope { where(active: true) }

  # constants come up next
  COLORS = %w(red green blue)

  # afterwards we put attr related macros
  attr_accessor :formatted_date_of_birth

  attr_accessible :login, :first_name, :last_name, :email, :password

  # Rails 4+ enums after attr macros
  enum gender: { female: 0, male: 1 }

  # followed by association macros
  belongs_to :country

  has_many :authentications, dependent: :destroy

  # and validation macros
  validates :email, presence: true
  validates :username, presence: true
  validates :username, uniqueness: { case_sensitive: false }
  validates :username, format: { with: /\A[A-Za-z][A-Za-z0-9._-]{2,19}\z/ }
  validates :password, format: { with: /\A\S{8,128}\z/, allow_nil: true }

  # next we have callbacks
  before_save :cook
  before_save :update_username_lower

  # other macros (like devise's) should be placed after the callbacks

  ...
end