How can I estimate the M/L ratio from a fitted SED on a galaxy?
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.