The simplest way is to set a "breakpoint" - open the module in the VBA
editor and mouseclick in the grey bar on the left of the code editor
window, next to the executable line of code where you want to set the
breakpoint. When you call the code, execution will stop at that point.
You can then use the various options in the Debug menu to singlestep
through the code, "step out" of a subroutine that you know to be
working correctly, "run to cursor" to run through a series of lines,
etc.