Flowchart of the sub-grid procedures applied to the SPH simulation to simulate line emission in post-processing. For each EAGLE simulation, we obtain the galaxy information from the database and we use the star and gas particle data to implement the ISM model. Next, we calculate physical properties in each phase to obtain the neutral clouds and DIG structures. We use CLOUDY cooling tables (Ploeckinger & Schaye 2020) to get the line luminosity for [C II]. We first calculate L_[C II],DIG and then calibrate the ionised gas using the predicted luminosities of the [N II] lines. We then obtain the total line luminosity from the luminosities of the different ISM phases. The dashed lines connect the gas environment to the ingredients of the model.