I would normally visit your home, work or place of storage to discuss your requirements, along with a selection of sample carpets.  In most cases a price will be calculated straight away.  
In most cases a price will be calculated straight away and upon acceptance a deposit will be required to cover material costs.
Either a template is made or the existing carpets can be taken away to be used to replicate a form.  
When the new carpets are ready, a mutually acceptable date and time will be established for fitting.  At which point the remaining payment will be required.