Getting the bus from LoWu would be a good option for most people.
We ended up on the 'ee' bus leaving at 8am from Mongkok. Getting through Huanggang was easy compared to past memories - and by 0930 we were on the bus going from Shenzhen towards Guangzhou South. The bus makes a couple of stops, and you might have to change to another bus to the final destination depending on the driver / number of passengers. Cantonese is spoken more than Mandarin; English wasn't. We arrived at Guangzhou South at 11:30. We were on the train heading north at 12:30 having spent around 10 minutes figuring out where / how to get tickets - we initially tried the automatic ticket machines, which have an English function, but then discovered that you needed a Chinese ID card to use them. At the ticket window we had to show our passports and the passport number was printed on our tickets.
If you are heading north, I think the high speed trains are increasingly an attractive option compared to flying.