Plug the output of the instrument or microphone into an open input of the audio interface. If you are plugging an instrument into the interface directly, a microphone will not be necessary.
Turn the volume on the instrument all the way up. To get the best possible sound and avoid hiss or crackling, make sure instruments are always set to maximum volume.
Set an open track in your recording software to "record mode." This will make the software able to receive the audio sent by the microphone or instrument.
Play the instrument. As you play the instrument, you will notice that the level meters in the recording software will begin to move. The level meter is the green bar that fluctuates up and down. The bar turns red when the sound being sent to the recording software is too loud. This is called overloading or digital clipping.
Adjust the gain. This is the most critical step in setting levels on an audio recording. The gain knob on the audio interface will control how high or low the signal level being sent to the recording software is. You want the signal level to be as high as possible without going into the red. A sound in the red will cause "digital clipping." Digital clipping is a very harsh sound that is very uncomfortable to listen to. It is a very common sign of an amateur recording. It is equally important that your levels are not set too low. Low levels lead to increased background noise and recording hiss. Setting your levels close to the red clipping level, but not exceeding it, will achieve the highest quality sound possible.
Press record. Once the levels are set properly, simply press the record button in your recording software and any sound being outputted to the audio interface will be recorded.