You can take mtr from Hung Hom to Luohu of Shenzhen (33HKD for 45 minutes). From Shenzhen to Guangzhou, bus and train all have their own advantages. Almost every bus station in Shenzhen's districts has buses to Guangzhou and the ticket price varies. Generaly it costs around 60 Yuan for 2 hours or so. If you want faster way, you can take bullet train at Shenzhen Railway Station and then get to Guangzhou one hour later, while the cost is a bit higher: 75/85 Yuan.