How do we design a LISN circuit for DC-DC converter?
On what criteria i should choose the circuit ?



If your measurement receiver is 50 ohms then it's likely that R2 (in
your 2nd diagram) is also 50 ohms.
C2 is chosen so that the lowest frequency of interest does not get attenuated by R2 too much
L1 is there to prevent the power supply shorting out the signal from the DUT so think about your lowest frequency of interest and make L1 large enough to prevent the power supply attenuating that signal by very much.
C1 and R1 are there to offer a low impedance at measurement frequencies thus, if the power supply (on its long cable) has a high impedance at high frequencies, there are no strange reflections that might affect how L1 works. For instance, if the power cable and supply together become an impedance transformer they might "project" a capacitance to be in series with the inductor (L1) that causes a resonance and ruins your measurement fidelity.


Also note that L1 is likely a series combination of mH, uH and possibly even nH in order to give a wideband impedance from kHz to GHz.