Hi Dmitry,
If we have obtained the residuals, then we can create a GARCH model and just estimate the variance equation, like
model = garch(1,1);
estimate(model, y);
Also, we can directly estimate an ARIMA model with GARCH errors, so that both the mean equation and the variable equation are estimated simultaneously. For example,
Mdl = arima(1,0,1);
Mdl.Variance = garch(1,1);
estimate(Mdl,y);
Regards,
Hang Qian