Copy and then edit s-param.cir. You will find this file in directory /examples/control_structs of the ngspice distribution.

The reference resistance (often called characteristic impedance) for the measurements is added as a parameter

.param Rbase=50

The bias voltages at the input and output ports of the circuit are set as parameters as well:

.param Vbias_in=1 Vbias_out=2

Place your circuit at the appropriate place in the input file, e.g. replacing the existing example circuits. The input port of your circuit has two nodes in, 0. The output port has the two nodes out, 0. The bias voltages are connected to your circuit via the resistances of value Rbase at the input and output respectively. This may be of importance for the operating point calculations if your circuit draws a large dc current.

Now edit the ac commands (see 17.5.1) according to the circuit provided, e.g.

ac lin 100 2.5MEG 250MEG $ use for Tschebyschef

Be careful to keep both ac lines in the .control ... .endc section the same and only change both in equal measure!

Select the plot commands (lin/log, or smithgrid) or the 'write to file' commands (write, wrdata, or wrs2p) according to your needs.

Run ngspice in interactive mode

ngspice s-param.cir