Connect an audio interface to your computer. Audio interfaces convert audio signals, such as the one from your guitar, into data that Sonar can interpret. They serve a similar function to studio mixing consoles, but on a smaller scale. Depending on the type of interface you have, connection is either with a Firewire or USB cable. Connect the cable to the relevant port on the side of your computer.
Connect a quarter-inch jack cable to the "Input" socket on the front of the audio interface. Press the "Inst" or "Instrument" button next to the jack to configure the input. Typically the default configuration for an audio interface input is line-level, which is suitable for recording synthesizers and keys, but not guitar.
Launch Sonar. Double-click the desktop icon. It may take a few seconds for the program to fully launch. The last edited recording session will automatically open. If this isn't the session you require, click "File" and select "New Session."
Click "File" again and select "New Audio Track." This opens a channel on the recording interface. Since you selected audio as opposed to MIDI, the audio interface is assigned as the default input device until you select another option.
Click the "Track View" button on the top of the Sonar interface.
Click "Media" and select "Audio Track Templates." This opens a library of recording preset folders. Each folder contains parameter settings suitable for a particular recording application. Click on "Guitar" and drag into the Track View screen. It will automatically configure the channel with the default guitar settings, optimizing it for recording.
Audition the guitar. Play it as loud as you intend to record and observe the volume unit, or VU meter on the front of the audio interface. If the LED goes red, the signal is too loud and will cause distortion when recorded. Turn down the "Gain" dial to lower in the input signal. Aim for as loud an input signal as possible before distortion.
Press "Record" and play your guitar riff.