My secretary does this online for me. You need to follow various prompts and questions that ask you things like destination station, ticket type, time etc. then input your passport (or Chinese ID) number. When all information is inputted, you then use a credit card to pay (my secretary uses my BOC credit card but it may be possible to use Unionpay if you don't have a CC). The website will generate a special code that is sent to your mobile but can also be printed from the computer. On the day of travel, you can go straight to a special pickup window at the railway station. Give them the generated code and your ID and they will issue the travel tickets.