Canooknic is right. Use that link to pay. I am assuming here that the application you are making is the 'outland' one, and that your girlfriend is a UK citizen - so you want her application to be processed via London. I've assumed this, since this thread is for outland apps via London. So first and foremost, send your application to Missisauga, Ontario. They will process step 1 (the assessment of you the sponsor) and providing that goes ok, they will send your girlfriends part to the London, UK office for processing. I would advise you to use a method which allows you to track the package and it;s arrival, as you will not get a note to tell you they have received it. First you will hear is that they have either approved or not approved you, which is likely to be 6-8 weeks away.
Regarding fees, do this online. The total cost is $1040 and is made up of the following components: Right of Permanent Residence (ROPR)fee $490, Family Class: Sponsorship Application (per application) $75, Principal Applicant $475. You do have the right to NOT pay the ROPR fee at this stage, however past experience from applicants on this forum and others suggests that this causes a delay, because they have to contact you to pay it. So if you are able to pay it now and get it out of the way, I would. Good luck!