To Fit a Cross-Peak or a Group of Cross-Peaks:
After processing the 2-D spectrum, create a Cross Peaks Manager and add your spectrum to its list of managed experiments. With the peak-picker tool, create a cross at the center of the peaks of interest. Assign those peaks to the respective atoms.
From the “Actions” menu of the Cross Peaks Manager, choose “Fitting Options”. These options belongs to the manager in use, not to the application. You might create many managers, each one with different options. More realistically, you can change these options before fitting different cross-peaks. If the options are not self-explanatory, read the help tips. If in doubt, keep the default options. All the parameters are integer values. If you don't want the peaks to move during fitting, set Max Motion = 0 ppb (1000 ppb = 1 ppm).
With the picker tool, select the cross-peaks to fit, then choose “Fit Volumes w. Gaussian Bells”. 2-D spectra are normally processed with strong weighting functions, therefore the natural (and uncomfortable) Lorentzian shape is rarely seen. You can select all the peaks of the spectrum, if you like. iNMR will split the calculation in parts, if the peaks are well separated.
All you can see, at the end of the operation, is a movement (if any) of the cross symbols (the assignments). To verify the goodness of the fit, select the Cross-Hair tool and move it over the plot. iNMR will show the cross-sections of the experimental spectrum and the fitting gaussian bells (in red, or the same color used for the scale labels). The more the two lines overlap, the better the fit.
If the first run does not satisfy you, try again, using the same or different options.
Some times it's better to restart from different intensity values and widths. Select one or more assignments with the picker tool and choose the command “Reset the Values”. iNMR will guess new starting values for you when you'll try to fit again the same peaks.