You only need to record a stereo track if you're using a stereo effect on the guitar, such as a ping-pong delay or a chorus. If you're going straight in or just using an amp sim, then you'll get the best results by recording mono.
1. Plug into the left channel of the card *
2. Select mono interleave for the audio track in SONAR
3. Select the card's left channel as the input source via the Input dropdown menu in the track header
* Assuming the card has separate left and right inputs, and not a single stereo jack. If all it offers is a stereo input, that will still work but you'll likely only get signal on one side. You can still select whichever side works in the track's Input dropdown list, and still record it to a mono audio track.