Been a while since I used that plugin, but I think after you install it you just add something like this to the controller or controllers that have actions that need SSL
class StoreController < ApplicationController
ssl_required :checkout, :pre_checkout, :pay_with_card, :place_order
ssl_allowed :place_express_order, :thank_you, :index
any actions that need ssl are listed after ssl_required, that way it automatically changes the url to https when someone goes to those actions.
If there are any actions if it doesn't matter if they use ssl place them after ssl_allowed and if someone navigates there using https it will stay https or if they get there using http it will stay http.
All other actions will redirect to http if someone gets there with https.