Indices : How-To : FontLab Interpolation Tutorial
Most discussions of methods to make bold letters use sans serif letters as an example. Since one of the great virtues of FontLab interpolation is that it preserves curve quality and character, I decided to use a serif example that includes curved stems and very irregular forms: Adobe Jenson Pro.
The first thing to do is to put the original outline in the mask layer (default Ctrl+M on Windows). During interpolation, FontLab will display both the result of everything you do and the original state, but having the initial glyph shape in the mask allows you to maintain a visual reference through multiple interpolation passes.