Dongguang to Guangzhou to Guilin by bullet train on April 6 2017

Hello

Please can someone give us some advise . We (4) Australians want some advise on how to travel from Dongguang to Guangzhou to Guilin By bullet. We intend to travel on April 6 2017. Should we book the tickets asap using a reputable website. Also we have relatives in Dongguang. Can we use our local relatives to book tickets on our behalf and is it allowed .? Which would you recommend ?