How to supervise an undergraduate student if I am a PhD student coming from a different field?
The situation may be challenging, but you don't need to be uncomfortable with it. In academic research, it is not uncommon that the one doing the actual project is more familiar with the material than the one supervising it. While it's probably rarely the case in an undergraduate - PhD student relation, it almost becomes the norm in the PhD student - professor relation, at least as the PhD student becomes more senior. Keep in mind that supervising is not only about teaching the material, but also about how to act and conduct research professionally. I hope you feel more confident about the latter aspect!

In your specific situation, there seem to be two possible steps if you feel you need to do something about it.


Talk to the student you supervise, explain that your background is in a different field, and openly state that you may not be able to fully help on the content side of the project. If the student is sufficiently confident in working more independently, you should be able to be more relaxed as well.
Otherwise, you may need to talk to your supervisor again and explain the problem. Maybe he can point out an additional member of the group who would be able to help with the aspects you can't cover.