Open Cubase SX3 and create a new music project. If using the Windows edition, click your "Start" menu in the lower-left corner of your desktop and select "Cubase SX3" from your list of programs. If using the Mac OS X edition, click your "Macintosh HD" in the upper-right corner of your desktop, select the "Applications" folder and double-click the Cubase icon. If you already have a project in progress, Cubase will load it automatically. If you want to start a new project, click "File" on the menu bar and select "New."
Connect your audio equipment. If using an audio interface (external sound card) with instrument ports, or any USB audio device, such as a USB guitar cable, you will need to integrate it with Cubase before you can begin recording. Click "Devices" on your menu bar and select "Device Setup" from the list. Select "VST Audiobay" from the left column, and use the menu to choose your interface or other USB device as an input source. If you cannot budget for an interface or USB instrument cables, you can use your computer's microphone as your input source, but your recording quality will suffer.
Create tracks for your recordings. Cubase SX3 functions as a digital multi-tracking recorder, allowing you to record songs one piece at a time, layering your guitars, bass, percussion, vocals and any other components that make up the collective parts of your mix. Your tracks look like long horizontal bars and make up the vast majority of your project window. To create a new audio track, click "Project" on the menu bar, hover your mouse over the "Add Track" option and click "Audio." Create a track for each layer of music you intend to record. For example, if you have just a guitar and a voice, you may only need two tracks. For an entire band, you will need many tracks.
Click the "Record" button on any blank track to begin recording. Your "Record" button appears on each track as a circle button with a smaller red circle inside. When you finish recording, click the button again. To layer the different parts of your song or other audio recording, align your recordings on their respective tracks. Your time line appears along the top of your window (resembling a ruler), and indicates the seconds of the song. So if you record a guitar riff on Track 1, beginning at the 30-second mark, and you record a bass line on Track 2, also beginning at the 30-second mark, your guitar and bass will play simultaneously.
Add effects to your tracks. In addition to recording raw audio, you can improve your sound by adding useful effects such as reverb, distortion and echo to individual tracks. Click the left column of any track to which you want to add an effect, and click the "Inserts" button on the left side of your project window. Select effects from your list.
Equalize your mix. To get the best sound quality from your audio project, you will want to edit your low, middle and high frequencies to perfection. Select a track and click the "Equalizers" button on the left side of your project window to access your multiband EQ window. You do not need to understand the precise frequency ranges to effectively equalize your music, but recognize simply that the low frequencies impact your bass sound and the high frequencies impact your treble sound. For example, if your bass guitar sounds thin, you can deepen it by lowering the bass frequencies. If you want to brighten your high-frequency sounds, like your rhythm guitars, synthesizer leads, brass instruments and flutes, raise your high frequency (treble ranges).
Save your Cubase audio project by clicking "File" on the menu bar and selecting "Save." To export your project as an audio file (such as an MP3), click "File," hover your mouse over the "Export" button and select "Audio Mixdown." Use the pop-up menu to choose an audio format, and enter a title for your song.