Google Analytics Cross Domain Conversion Tracking

Hey, We already had a setup to track conversions. Recently we upgraded our website and lost all codes. So your job is that you have to setup conversion tracking properly in cross domain.. Visitor comes to product sites, goes to company site to make purchase and once purchase is done he comes back to product site. so the code should handle cross domain stats. All the works related to script and in google analytics side should be managed by you. You need to setup everything. few codes are already available. its jsut 10-15 mins work if you know it well. dont bid if you havent worked on it and i really have no time to waste of your copy paste messages.. bid only if you have worked on this. no fresher please! I Need Proper Documentation as well.