You can set up multiple layers, like light and bold, and generate interpolated instances.
Go to Font Info and select Master pane.
For each Master, choose a appropriate value from the popup and set a distinct value for the weight. We suggest the width of the standard vertical stem. (e.g. take it from the "n").
You can set up a font with up to 4 masters. This can be three masters in a row (like Light, SemiBold and Black), In a "triangle" (Light, Bold and Condensed) or in a spare (Light, Bold, LightCondensed and BoldCondensed).
Got to the Instances pane.
Add a instance with the "+" button in the bottom left corner, and fill in the information for Property and Grouping according to the planed instance.
Set the interpolation value. If you used the stem values for the masters, you can put in the desired stem width for the instance.