Emerald Lance, said:
Hex editing can indeed be a daunting task for beginners. When I first started, I remember that it just looked like a swirl of coded nonsense. But there are only two things you need to get started: a hex editor, and knowledge on how to count in hex.
If you're talking about modding 360 game saves, programs like Modio and Horizon have a hex editor built in, so we'll skip that part. Assuming you don't know how to count in hex, this is the general basic concept. When you normally count, you go 0-9, and then it starts again as the zero of the next series of numbers, in this case 10-19, then 20-29, and so on. In hex, you count from 0-F; instead of going on to 10 after 9, you go from 9 to A. In hex, A means 10, and F means 15, so in hex 10 means 16, and so on. A lot of people starting off have a bit of trouble understanding at first, but you'll get it sooner or later; I learned by trying to count in hex in my everyday life, and by studying how color sliders that went up to 255 were different than the ones that went to FF (there is no difference, just decimal vs hex). Of course, this is assuming you don't already know how to count in hex.
While much of the hexing process is just changing values, you gotta know where those values are in in order to change them. Every game is different, so this is important. There are two methods you should be familiar with: Ctrl+F, and comparison. Let's say you want to edit a character's HP stat. While playing the game, take note of what it is. Let's say it's 590. Right it down and use a hex calculator (Windows has one that comes with it) to find out what it is n hex; in this case, 590 = 024E. Remember to add a 0 at the beginning of values that have an odd number of digits, since hex is in bytes (two digit values) and it helps make sure you don't get a bunch of values you don't want. Now, in your hex editor, hold the Ctrl key and press F, then type in the hex value you want to search for.
It isn't uncommon for the search to turn up multiple hit, especially when searching for lower (and by proxy, more common) values. When this happens, you need a comparison. Play the game normally and do something to change the value you want to edit, in HP's case I guess level up once. Take note of the new HP value in hex, and repeat the Ctrl+F process with this new save. Let's assume you're new hex number is 02D6. Since the only instance of 024E that changed was HP, then that should be the only instance that is now 02D6. Look for 02D6 now, and find the place where it sits that 024E used to sit. It's easiest to do this when you have both old save and new save open at once; you can also write down each address if there aren't a lot of instances. Once you find the value (and know that it's HP) change it to whatever you like. It can be sometimes risky to edit games with a maximum value of 9999 (270F in hex) to higher than their maximum, but in most games it works fine. Now just remember to resign/rehash the save (again, programs like Modio and Horizon do this) and inject it into your drive.
There are a few things that can go wrong with just simple hexing. While most older games can just be edited easy, newer or more popular games often have some measures put in place by the developer just to make sure it isn't hacked. The game save could be compressed or encrypted (will essentially show gibberish until decompressed or decrypted) or could have a checksum (a little line of encrypted code that changes every time values change during gameplay; if the values don't match the checksum, you get a corrupt save). Just remember that not everything can be edited with just hex knowledge alone.
Believe it or not, this is all there is to know (that can be taught) about hexing. Everything else is just picked up as you go, including better understanding of hex as a language (as opposed to just a number system). I hope this helps, and I hope it gets you started on your hacking way.