In the Input parameters section of the dialog, select the nodes which define the free edge to stitch. Specify the stitch tolerance; nodes closer than this will be merged.
In the Selection option section of the dialog, choose whether you want to use a filter.
The distance between the nodes to be merged is the tolerance, and the default tolerance is 0.1 mm. You can specify a tolerance of your choice if you want.
Either accept the default (0.1mm), or select Specify and enter the required stitch tolerance.