How can I estimate the M/L ratio from a fitted SED on a galaxy? This answer (here in the forum) provides background to my question: https://astronomy.stackexchange.com/a/5953

I am trying to understand the process in which a stellar population synthesis (SPS) is created, in order to calculate the stellar mass of a galaxy. (This is my main goal: understand how the stellar mass is calculated via SPS and broadband photometry)(Section 2.5:  https://ned.ipac.caltech.edu/level5/Sept14/Conroy/Conroy2.html)

Ok.
I have already understood the process to obtain a stellar population library (SPL). Such SPL will be fitted to observational data in order to create a SED (Spectral Energy Distribution)

Ok. Now I have the fitted SED on a galaxy, here are my questions:


How can I estimate the M/L ratio of that galaxy?
How can I estimate the L?


I feel it is an easy and direct answer, but I don't get it.
The luminosity L isn't estimated, it's observed. You make models of stellar populations with many different parameters and those give you a set of SEDs. For each set of parameters, you will have an SED covering a range of wavelengths (a continuous line).

From your observations, you will have an SED of a few points depending on how many wavelengths you observed. You use $\chi^{2}$ fitting or some other technique to compare all your model lines with those few points and see which fits best. This gives you the best fit value for the parameters in your model, one of which is M/L.

You have the actual luminosity of the galaxy from the observations (at some wavelength) so you multiply the estimated M/L by the measured L to get an estimate of M.