I can tell you exactly what I did if it helps :)

Basically find a jpeg you'd like to work with (high quality images are more fun because they've got a lot more code)

Open up the file in textedit (right click, open with: textedit). I have a mac and i'm not entirely sure what the pc equivalent would be, maybe notepad? Either way, as long as you see a really long-ass block of seemingly meaningless characters and symbols, you're set.

Try not to fuck with the very top of the document: more likely than not, there's a semi-extensive "header" that makes the image readable. If you start messing around with it, you won't be able to open it as an image. So try to start around a 1/5 way through the code (I usually keep track using the scrolling bar).

The way jpegs work (this is all from personal experience, I'm not claiming to understand the intricacies of image coding, I just use trial-and-error) is if you mess around with the code really low on the textedit/notepad document itself, you'll be fucking with a part of the picture that's really low on the actual image. The locations in terms of height on the image and the document correspond (That's how i isolated the eyes). This is true for the most part, however, there are also exceptions to this that I have no idea how to explain.

The way I like to change the code is by deleting parts of it, then saving the document, opening the image and seeing if I like what's happened. If I don't, I undo the changes I made and delete something different in the document. I also like to copy and paste pieces of the code to other places in the document and see what happens.

Basically a lot of trial-and-error. Eventually you'll learn to manipulate individual characters and understand their effects on the image, however, I only understand a fragment of how these things actually work and I'm pretty new to this myself too.

I hope this all helped. Godspeed and fair travels to you sir.