The distance between Hong Kong and Guangzhou is only 130km
The best way to get to GZ from HK is by high speed train, which takes only 1 hour and costs ¥215 (~$31) one way.
The train departs from the very centrally locatedWest Kowloon Railway Station (西九龍火車站), just a 10-20 minute metro ride from the financial district. You can get there by metro at either:
Kowloon Station on the Airport Express line and the Tung Chung Line
Go to level B1 (first level below ground) to buy tickets and go through security. From there you'll need to go down to B3 to clear customs before going to the platforms on B4.
The service is very frequent. There's more than 30 trains every day running between 8am and 7pm, once every 20-40 minutes. I suggest using Ctripto check the schedule and book tickets
The train will get you to Guangzhou South Railway Station, which is on subway line 2 about 40 minutes from the downtown area
You clear Chinese customs at West Kowloon station in Hong Kong, not Guangzhou, so you need to have your visa ready before you go. Once you're on the train, you're basically in China and you don't need to do anything else once you get off the train in GZ.