How to manage internal dependencies?

Question

Programming

Greetings!
I'm not sure about the terminology, but I think I phrased it correctly.
From time to time I write small on a cosmic scale, but quite extensive in terms of macro writing projects under Office in VBA. This immediately determines the absence of any detailed documentation on the product.
Sometimes you have to make small additions-changes to the code, which, in turn, can lead to unpredictable consequences for the entire system. Since there is not much time for testing either, the work scenario is as follows: I made an edit, waited until the user came running in horror from the fact that something had stopped working, fixed the breakdown.
I'm interested in the following: is there some proven and simple way to document these internal connections clearly and effectively enough to know for sure: corrected here - see how it responds there. I have an idea about input-output-control schemes, the main question is how to apply it most simply. Maybe there is an algorithm for reliably compiling such a map, or what kind of checklist?
I would be grateful for any advice and comments.