Fitting a spread into ARIMA AR(1)

I'm a newbie to econometrics. I've simply ran a regression and have coefficient values of the variables. I'm running a regression for a crypto data, and I've gotten the Spread of the variables. To forecast, I'll need to fit this spread into ARIMA AR(1) process after finding the best fit is AR(1).

I don't understand how to do this fitting, please help. I'm sorry my language or writing may not fully communicate, but I know the Profs here understand how to pick what I'm trying to say.