Creating child theme after already editing parent

I am new to WP and learning as I go along.

I’m using a theme from inkthemes.com which I have made some tweaks to e.g changing colors of text and menus. I have only just read about child themes and think it best that I create one.

I’m pretty sure that I understand how to create a child theme, but how can I do this and still keep the changes I have already made to the parent theme?

Am I able to just copy and save all of the modified theme files, re-install the theme, create a child theme, use a text editor to find the differences between the re-installed files and the previously modified files and then add them to the child theme?

Please excuse me if any of what I’ve asked either makes no sense or is technically impossible. I’m still learning the ropes and trying to get to grips with it.

Any help in this matter would be greatly appreciated.