Will you arrive at Macau by air? If so, I think you could take the inter-city coaches from Macau Airport to arrive in Guangzhou directly. The coaches leave from the airport respectively at 12:00, 13:00, 17:30 and 18:45. The fare is Mop150.

If the bus time is not proper, please take the city-bus AP1 from Macau Airport to reach the Gongbei Port firstly. The journey is about one hour and costs Mop5. Walk through the Gongbei Port, you could go downstairs by elevator to the Qiguan Bus Station (B2) here. There are lots of buses departing to Guangzhou from this station everyday. The bus fare should be around 65RMB.