From your Mac, how you can connect to your remote Ubuntu desktop?
Note: This is also applicable to other Linux desktop with some variations in step 1-3
Open Remote Desktop Preferences
System -> Preferences -> Remote Desktop
Enable Sharing by desired
Select Security as desired
In your Mac, download and install any VNC client for Mac; I recommend using JollysFastVNC as it is really fast and joy to use :)
Using your VNC client in Mac, connect to your remote Linux desktop.
How? Well, as a Mac user, I'm sure you know to do this :)