Make sure you have the ASIO drivers for your UR28M installed. In Cubase, go to devices/device setup/VST Sound system.
Select your steinberg ASIO driver in the dropdown menu.
Next, press F4 to open the VST connections window. Assign busses to the inputs and outputs on your UR28M.
Now you should be able to create an audio track and select an input for it. Press the round button with a dot on it to enable that track for recording. Press the round button with the speaker to output the recorded sound directly to your speakers while recording. Press the record button on the transport panel to start recording. To listen to your recording, disable the monitor button again (the round one with the speaker on it) and hit play.
That should be the basics for audio recording.