How to detrend the prices

Hi guys, would you please help me in this matter.

I have a NK model with CPI and commodity prices in level. What I know is Price is detrended by M/A. However I don’t have Money in my model. Money is just unit of account and represented by interest rate in my model.
Would you please somebody give me a clue or a reference on this? Many thanks.

You have to express everything in inflation rates and relative prices. See for example