what steps and software do you use for merging files?

There are often some files overwritted, such as tpl_product_info_display.php, html_header.php when install new add-ons.

In that case , you have to merge the file and it's really a difficult thing to do.

what steps and software do you use for merging files?

Thank you very much

I use Compare It! to compare the current modified file to the unmodified file in a stock Zencart installation, and make a note of the line numbers involved and to get an idea of what the modification was intended to do.

Then I compare the current modified file with the file from the mod I'm trying to install. If the modified sections don't overlap (that is, the edited files aren't changing the same parts of the code), I add the edits from the new mod file to the existing modifed file.

If the new file does modify some of the same code as the existing edited file, I usually give up (choose between the two mods), as I'm not programmer enough to wrangle the code.