Creating First Mod, Need Advice??

Hi! These are the basic steps to clone a standalone item (meaning it will appear as a new item and not under the original's hair thumbnail). If you clone a hair from X expansion pack then your object will require that said EP. This is in case you want to share it, so you should let other people know.

sims4studio.com/thread/12/tutorial-standalone-recolor-custom-thumbnails

Once you export your texture, you open it in your image editor of choice and then import it back.
If you're going to edit it in blender, then Export the mesh from the "Mesh" tab, edit it as you would and then import it back the same way.

As for your specific project, bare in mind that EA's hair usually have their strands mapped overlapping one another. Meaning that, if you were to recolor a strand in the texture, probably more than 1 would change. So if you were to want just the ones from your image, you'd have to move all those strands' UV parts to the same spot (ALWAYS inside the original's texture boundaries).

If you want more tutorials, i suggest looking on youtube. There's really a ton.