Regal Riverside Hong Kong hotel to Guangzhou

Firstly, you could take the hotel's free bus to Sha Tin train station and then take the KCR East Rail train (30minutes) to Luohu border. After go through the border, you could enter into Shenzhen Railway Station and then take an express train to arrive in Guangzhou in 70 minutes.
The KCR train fare is HKD26.5.
The train to Guangzhou will cost you CNY75.

On the other hand, you could take the train (15 minutes&HKD5.5) from Sha Tin to Hong Kong Hung Hom train station, and then proceed to Guangzhou East Railway Station directly by train (2 hours&HKD190).