If it's not much, just call and say that you owe someone (amount) money today, then sign your name and attach a copy of your ID card. Chinese is fine. Write Japanese. Can you understand?

Let the other party write. Write it in front of you and sign it in front of you.

A lawyer witness is equivalent to a lawyer's certificate. Note that lawyers are witnesses, not notaries. Notarization is done by notaries. Witness fee 100 varies from place to place.