The debugger is very simple, and is the same environment you're using to type
your code now.

Simply put a breakpoint on the first statement inside your function  (it won't
put breakpoints on statements like DIM statements, so it might not be exactly
the first statement). Then whenever your function runs next time, it will break
at that point and display the line of code. You then have options to step
through the code and you can view the contents of local variables. The
short-cut key to put a breakpoint on a line is F9.