I found it hard to do this with sfntly, and I'm not sure if it's possible, as sfntly wasn't made for that (see this discussion https://groups.google.com/forum/m/#!topic/sfntly-users/V0a6HzzDN2w). You might want to look at doubletype (http://sourceforge.net/projects/doubletype/) or a Processing (http://processing.org) library I wrote to create ttf and woff files and define each glyph shape: http://code.andreaskoller.com/libraries/fontastic
Sorry for the shameless self-promotion of my library but I actually think it might be a good start. If you're a more experienced programmer then I'm sure you'll find a better way. But be warned, Fontastic currently lacks support for special characters and typographic necessities such as ligatures or kerning.
Best,
Andreas
Regards,
Sumit Nalawade