Here's the basics on how to use it.
First, you have to create a reference to a number. use %reference = f_makenum("hexadecimal_here");
to create a reference to a number. The hexadecimal will be put into the number, so if you do %reference = f_makenum("1234");
it'll give you a reference to a number that contains the decimal value 4660.
Then, to add two numbers, send the f_add, f_sub, etc functions two number references (they have to be different!) f_add(%ref1, %ref2);
(notice i'm not using the return value) will add the values %ref1 and %ref2 refer to, and place the result in %ref1.