I just added a step to my checkout process, and it was definitely a
matter of piecing together a lot of info from a lot of sources.  The
guides don't seem to tell the whole story, and in some cases appear to
be wrong.  I thought I'd list my process here, in the hopes it helps
someone, and in case I've done something wrong!

So...step 1 - redefine the state machine, as described in the guide.
However, I had to add two additional lines of code:

      StateMachine::Machine.ignore_method_conflicts = true
      Spree::Order.state_machines.clear

The first to suppress the conflict warnings, and the second to clear
the old state machine.  So my code in app/models/order_decorator.rb
for this looks like (I added a "dates" step to the order process):

Spree::Order.class_eval do

  StateMachine::Machine.ignore_method_conflicts = true

  Spree::Order.state_machines.clear

  # order state machine (see http://github.com/pluginaweek/state_machine/tree/master
for details)
  state_machine :initial => 'cart', :use_transactions => false do

    event :next do
      transition :from => 'cart',     :to => 'address'
      transition :from => 'address',  :to => 'delivery'
      transition :from => 'delivery', :to => 'dates'
      transition :from => 'dates',    :to => 'payment', :if
=> :payment_required?
      transition :from => 'dates',    :to => 'complete'
      transition :from => 'confirm',  :to => 'complete'

      # note: some payment methods will not support a confirm step
      transition :from => 'payment',  :to => 'confirm',
                                      :if => Proc.new { |order|
order.payment_method &&
order.payment_method.payment_profiles_supported? }

      transition :from => 'payment', :to => 'complete'
    end

    event :cancel do
      transition :to => 'canceled', :if => :allow_cancel?
    end
    event :return do
      transition :to => 'returned', :from => 'awaiting_return'
    end
    event :resume do
      transition :to => 'resumed', :from => 'canceled', :if
=> :allow_resume?
    end
    event :authorize_return do
      transition :to => 'awaiting_return'
    end

    before_transition :to => 'complete' do |order|
      begin
        order.process_payments!
      rescue Core::GatewayError
        if Spree::Config[:allow_checkout_on_gateway_error]
          true
        else
          false
        end
      end
    end

    before_transition :to => ['delivery'] do |order|
      order.shipments.each { |s| s.destroy unless
s.shipping_method.available_to_order?(order) }
    end

    after_transition :to => 'complete', :do => :finalize!
    after_transition :to => 'delivery', :do => :create_tax_charge!
    after_transition :to => 'payment',  :do => :create_shipment!
    after_transition :to => 'resumed',  :do => :after_resume
    after_transition :to => 'canceled', :do => :after_cancel

  end

end

Ok...then, it doesn't work.  I had to add a app/helpers/
checkout_helper_decorator.rb, with the following:

module Spree
  module CheckoutHelper
    def checkout_states
      if @order.payment and
@order.payment.payment_method.payment_profiles_supported?
        %w(address delivery dates payment confirm complete)
      else
        %w(address delivery dates payment complete)
      end
    end
  end
end

Basically, I needed to add in my new step into the list.

Then, I needed to add a translation for my new step; contrary to what
the guide says, this needed to be something like

en:
  order_state:
    dates: Dates

not "checkout_states" as indicated in the guide.

Then I needed a view, which had to be in app/views/spree/checkout/
_dates.html.erb.  Leaving out the "spree" folder in the middle of that
path would not work.

Anyway, if I've done something silly let me know...and hopefully this
will help someone else from having to spend so much time trying to do
this!

Greg