I'm another one who needs a step by step guide for this. 
Thanks trepae!
Do you need to start a new game to make the changes? Nothing I changed seems to have actually changed ingame.
You'll need to do a few things beyond trepae's instructions. After you've modified your .ini file to your liking:
1. go
here and download modpatcher.
2. unzip the contents to your game installation directory's binaries\win32 subfolder.
-This should be where your xcomgame.exe is.
-Make sure to make a backup of xcomgame.exe in case you need to start over.
-make sure modpatcher.exe and patcher.ini are in this folder and not in a subfolder of their own.
-the defaultgamecore.mod that comes with modpatcher is unnecessary as we will be using our own, and the prepackaged one has the default game settings on it anyway.
3. copy your modified defaultgamecore.ini from its folder into your xcom\binaries\win32 folder
4. change the file extension on your defaultgamecore.ini from .ini to .mod
-windows will most likely warn you that changing the file extensions could potentially cause some really bad juju. Just proceed, it'll be fine. Probably.
5. run the patcher.bat file that you got with modpatcher
-this will modify xcomgame.exe to use the settings from your customized .mod file.
6. make sure steam is in offline mode. If it is not, switch to offline mode now.
-I am not savvy enough to understand the technical reasons behind this, but from my limited understanding the game actually derives many settings from an external source, maybe on a Firaxis server hidden deep within their ant farm lair somewhere. So if you try to play the game while steam is online, many of your custom settings will revert to default settings.
7. start the game and enjoy, your settings should be in effect now.
Hope this helps!
TIPS:
-a good way to verify if you're doing this correctly is to change the base damage of the Assault Rifle to something other than the default (which is 3 I think. I know ARs do 4 dmg default ingame, but for some reason the game adds 1 dmg to whatever you set it to in the .ini. perhaps someone else knows why). Then start a new game with tutorial disabled. Make contact with the enemy in the first mission, target an enemy with a rookie (DON'T shoot, just target one), and hit f1. On the statistical breakdown window that just popped up, make sure that the rifle damage was changed (it shouldn't be 4 anymore, it should be whatever you set it to +1). If you've changed the default offensive/accuracy score for rookies or their starting will, this would also be where you'd check that.
-once you've got your .mod in place, you can edit that directly with XCOM Meddler instead of editing the .ini, copying it over, and changing the extension every time.
-remember that you have to run patcher.bat every time you change your .mod settings in order for the changes to take effect.
-as others have said, some changes will require a fresh game to take effect. The weapon damage does not, though - it will always be what you set it to as long as you did all this right, hence the reason why it makes a good test.
OK, I can't get this to work on my end. I think I'm missing a step somewhere:
1) open defaultgamecore.mod (located in binaries/win32 folder) the with meddler, edit/save.
2) re-open .mod file again to confirm that it did, in fact save all changes.
3) run the .bat from modpatcher (I've tried the current download posted from a forum here on the nexus, as well as the one included with the battlescape mod)
4) start the game
I know it has to update the .ini file embedded in the games .exe, and I figure this is what the modpatcher .bat file is doing (besides creating copies of the original exe) but the changes aren't showing up in game. What am I doing wrong?
Make sure you're playing with steam in offline mode.
Edited by iRhuel, 24 November 2012 - 02:53 AM.