How to determine the coefficients of an ARMA filter

I'm new to filter design. I consider a two-tone input signal which is then alterred by a nonlinear device. I now have the input and the output signal, both of which are cosines, but in vector form. With this I mean I am given the samples of the cosines. In MATLAB, I want to design an ARMA filter, which will be put before the nonlinear device. How can I determine the coefficients of the filter? What are the necessary steps when designing the filter?