The program is über complicated and I'm still trying to figure it out. There are some good instructions on the Font Lab Web site.
A very inexpensive way to make a font from your own handwriting is to use a website called YourFonts. For only $9.95 you can turn your own handwriting into a font. This is a real deal, since the FontLab program is $649 US dollars. You can try it first to make sure you like it, and purchase your new font when you're ready.
These directions are very general, but at least they give you an idea of the font making process.
Modifying a font is a lot easier than creating a new one from scratch, because you just open up an existing font in Fontographer or FontLab and edit it. You then save the changes to the font, and you've created a new font from an exisiting one.