A very loooooooooooooong time ago I used to always use the ammonia because those who wrote about molding balsa said to.  No more, I've not used it for the last 15 or more years and have molded balsa on every stunter I've built except for the profiles.  It just ain't needed!
I have also found that soaking the balsa for no more than a half hour is sufficient.
Blessin's,
Will

Ammonia softens lignin and subrin, which balsa has little or none of.  The problem is getting the balsa whetted.  A tiny bit of simple green or similar will help in the process.

Steve: The best way is to get Windy's DVD's on molding Balsa. He shows how you carve the top block then mold balsa around it.

Or Robins view from Bob Hunt. Thats how I leared. Great stuff!

That's what i got. But in the video the plans have have formers for molding the top and bottom blocks.

Agreed. Lofting is the best method, whether by the old historic methods, modern CAD methods, or whatever... (As I learned it about 50 years ago, the term 'lofting' was related to laying out - full-size - the curves of boat and ship hulls. BIG pieces: needed to work in a big open space, like a loft? Also used later for early aircraft full-size layouts...)

To Reply#4, Steve - a 'practical' way might be to build the forming buck with the intended formers in place at the desired positions (if the plans don't locate them for you.) You could tack-glue the formers with a dot of, say, SIG Cement, which acetone or dope thinner softens to let you separate them from the blocks. Or, you could 'build' the buck onto a keel, with the separate short blocks between the bulkheads securely mounted to the keel. Then carve away, just as you would have from solid block in the first place.

Remember to make the buck width and height undersize by the thickness of the wood to be formed... of course.