IPS is just a patching format. It takes changes and applies them to another file. It has no special awareness or nor relevance to any kind of hacking.
There are many tools to make them
http://www.romhacking.net/utilities/13/ is the one I like.
That said IPS is an awful patching format and worse for the 3ds. I am quite shocked I have seen it used, and it is not a coincidence that the list of things you have to do to make effective use of it is long and tedious.
First it will need to be applied to the decrypted ROM. This will be the case for all formats though as soft decoding is not an option, and I have certainly not seen anybody implement a means otherwise.
The size limit is around 16 megabytes (one of the reasons you have to extract existing files in the stuff already out there).
It does not handle data shuffled around (happens all the time on modern systems, not so much on the NES and SNES where the format rose to prominence).
There are many more modern methods that I would suggest over IPS. Xdelta (
http://xdelta.org/ ) and bsdiff being the big two but there are others that would also work.
It is always on, or at least I assume none of the CFWs/loaders have implemented soft patching of IPS (smea's HANS stuff is soft replacement but it is not IPS). This does not stop you from coding a button activator of some form into the cheat though, if you can pull off some of the things I am about to cover it should in turn be obvious how to make such a thing.
--- start of the hacker talk ---
How to patch a cheat in varies depending upon what it is and how it works. If the cheat attacks the binary in memory (loads of DS ones do this, I have not analysed the 3ds scene much though) then it is simple enough and you just need an uncompressed version of the binary to mod (and it may not even be compressed to begin with). This is about the only scenario in which
If the cheat edits memory then it gets harder. Older systems have means to hardpatch cheats in (things like GBAATM, GABsharky and DSATM) but nobody has gone there for the 3ds yet. You can still use various means though. There are two main ones
1) You find the instructions responsible for doing the thing you want changed and change them in turn (turn a subtract from lives counter into an add or something). Hard if there are many things changing it -- the classic thought experiment is to consider all the ways to die in mario and thus all the various ways the lives counter could go down. In the case of always catch there will probably be a long calculation (
http://bulbapedia.bulbagarden.net/wiki/Catch_rate ) that ends with a yes or no and you change the result to always be yes or it to always take the yes path.
1a) For a pokemon hack you could probably also work with the catch rate stuff above to see if you can edit the pokemon to always be caught by simple maths alone. Not a great path for this but still an option.
2) You make a cheat engine of your own. 1) is the human way of doing it for individual games. Cheat devices will instead attack a routine that runs all the time within a game (typically the vblank interrupt cycle) and insert an instruction to hold a value at a certain value (or maybe something slightly more complex but still stemming from the same thing). This is why infinite life cheats sometimes fail if the amount of damage done is more than max health. Find the vblank routine and a space to crowbar a memory write into it and then you have your cheat. The button activator stuff means rather than a simple write you first check to see if a button is pressed and do the cheat, or you write a cheat active flag to memory after checking buttons and then have a disable command and have the cheat instead check the flag in memory and react accordingly.
The 3ds offers a slight tweak on 2) here to become like the PC in about 1990 as it is actually a real operating system and halfway modern system design. This means if you can attack the firmware you can have the firmware, which is running all the time on the system, do your dirty work and fiddle with a memory location every so often instead of having to wedge it into the game code. This will require everybody using the hack to have a similar cfw setup where patching the game will merely require them have the option to run a patched game.
If a cheat edits the save (what the earlier cheat devices on the 3ds did and what a few of other things do on older systems) then there are things you can do to turn it into memory and instruction cheats but that is getting off topic. Short version is you do a tweak on the classic change one variable and scan again cheat finding method. If you want to get really crazy you can edit the save from the game level too.
