Basically, when I make a change, I print out (copy paste to notepad) the template. I highlight the changes made. When I upgrade, I take a look at the new template and see if the changes are needed (sometimes if it's a fix, the change has been made already - ie market price).
I then read over the code and find the appropriate location for the changes. You do need to know how to read PHP. I may not be able to write PHP from scratch, but I have gotten familiar with the syntax and method of code. Typically I can follow what the code is doing and how. I can write a few lines of code, but simply can't write a full program or add a full mod.
So, once you locate the lines you need to change, you send it. Remember, since you are working on a non-live site, if you make a mistake in a template, you can always revert the template back to it's original.
It simply is not smart to make major changes to a live cart. Therefore, now in 3.4.* there is a WONDERFUL feature where you can click to take the cart down. Now when you make changes to a live cart, you are not doing it while someone is shopping. I get a real laugh when ever I remember the time I was making major changes to the color scheme of the cart. While playing with the colors, and I mean really messing with them!, I found out later someone was actually shopping at the time! I giggle everytime I think about it! Imagine shopping on a site and the colors keep going wacko! Every few pages the whole color scheme completely changes! Oh that must have been really weird. I wonder if she thought it was on her computers end!?
There were times when I mistakenly made the text the same color as the background! OH! suddenly no text! Text pretending to be ghosts! Then suddenly the text comes back in a different color! (snicker!!) I learned never run tests on colors on the live site! Test the color scheme first on the test site or you may have customers checking themselves into a rest home.
