Select which parameters, click Fit and wait.
The fit does a gradient descent search through the fit parameter space using a Levensburg-Marquardt non-linear least squares update procedure. Each step of the fit it updates the new table parameters, profile and reflectivity curve. At the end of the fit it estimates the uncertainty in each parameter.
You can click Stop, and at the end of the next update cycle the fit will terminate with the current parameters. Click Revert to restore the parameters to what they were before the Fit. Save your parameter sets frequently with >File >Save As....
Fit parameters are selected with the mouse. Click and drag to select a block of parameters. Press Ctrl before selecting if you want to add parameters to the selection. If you are using constraints you may need extra parameters which are not part of your layer structure. Any parameter not used in the layer table is available. [MLayer uses the layer after the last layer in the top and middle regions for its own purposes, so don't use those.]
At present there is no way to save the parameter values or their uncertainties other than selecting and pasting them into an editor.