I need to get from Hong Kong Airport to Airport Dongguan Check-in can you please advise how?

Do you wanna say DG City Terminal?
If yes, there are several direct long-distance buses at the airport that can take you the destination. They depart at 07:10, 08:10, 09:10, 10:10, 12:10, 14:10, 16:25, 17:10, 18:55 and 19:50. The ticket fare is 160 HKD per person. And it takes around 2 hours on the way.

Where do you go to buy the tickets and catch the bus within the airport?

The boarding point is located at Coach Station, Terminal 2.
And the ticketing points are located at Counter C10-13,Coach Station, Terminal 2.
You can easily find those places accordind to the clear English signs.
Don't worry. :-)