The algorithm of multiple PSFs by nonlinear least-squares in the STORM super-resolution localization

Recently, I have wrote a SOTRM procedure to achieve high localization accuracy through multiple-PSFs ,the procedure is to grouping first, then determine the number of fluorescence points in each group. After that, using Gaussian functions to fit the group(the number of Gaussian functions is equal to that of fluorescence points), so I will have the coordinates and variance of each phosphor dot , as well as the intensity values.But I do not know how to fit by nonlinear least-squares. Anyone who has wrote similar procedures please help,thank you!