Theses steps assume that IIS is installed on your server and a website has already been pre-configured.
Open Server Manager -> Click Tools -> Click Internet Information Services (IIS) Manager
Select your web server
Double click on Server Certificates
On the right hand menu, select 'Create Certificate Request'
Fill in the following information
Common name: the url you want to apply the SSL certificate to (i.e www.snapwire.co.uk)
Organization: the name of your company
Organizational unit: your company's unit or department
State/Province: Your geographical location
Country/Region: i.e. GB for Great Britain
Now select the Cryptographic Service Provider and the Bit Length of your security certificate (you will be able to gather this information from your security provider, i.e. the company you are purchasing the SSL certificate from)
Name and Save the file, aka a CSR or Ceriticate Signing Request (click the ellipses on the right to specifiy a location
Open Windows Explorer and navigate to the folder you saved the certifcate in.
You will need to provide this certificate to your security provider / certificate authority. They will provide you a new server certificate so that you can complete your request.