How do I stop steps registering while playing the guitar?

Wear it on your left wrist, if possible. Depending on your playing style, it should be less steps, but the accelerometer might still register steps. Alternatively, try wearing on your ankle.

My Garmin Watch is Not Accurately Counting Steps

Another solution is starting an activity suppressing the steps detection, like for example Indoor Cycling, Rowing, and others (full list at Why am I Not Gaining Steps During Certain Activities | Garmin Customer Support, but in fact it may vary from device to device, so you have to test which activity suppresses the steps on yours). At the end you can discard the activity. Or you can keep it and rename it to Guitar Playing, if you want to keep the log.