Wrote the guide on steam, too lazy to reproduce it in full here, so here are some primers.
The full guide:
Modding Alien: Isolation - The Basics.
Backup XML + BML files inside the game installation path:
They do not take up much space, so keeping a backup of the entire folders of alienconfigs, weapon_info, chr_info, difficultysettings, view_cone_sets, aim_ik_data, behavior, binary_behavior, material_data and a few others, will only take up an extra few megabytes, nothing to worry about hogging space.
The tool is simple to use. A precaution to take before anything else is to just right-click on the executable file, select properties, go to the compatibility tab, just to be safe click the "change settings for all users" and then tick the box for "Run this program as an administrator" and of course apply/ok for it to set.
Whenever you need to convert a .BML file to an .XML file, or the other way around, simply drag and drop the file in question on top of the AlienBML.exe, it then creates the converted version, and adds a "_noconflict"-suffix after each time it creates a new file, to avoid overwriting any existing files of the same name. So you need to drag-and-drop say for example the gbl_item.bml file onto the AlienBML.exe, it creates a file called gbl_item_noconflict.xml, you can open it up in notepad and change things, save, then pop the edited gbl_item_noconflict.xml on the AlienBML.exe to create a bml file again of your edited xml file now called gbl_item_noconflict_noconflict.bml, so next you need to rename the new bml file back to the original by removing the duplicated "_noconflict"-suffixes, and lastly put the modded gbl_item.bml back into the the data folder:
Overwriting the existing file when prompted.
(Naturally for other files another folder than the base DATA folder.)
In the base DATA folder exists the GBL_ITEM.BML file which holds such basic things as how much of each weapon ammo/batteries/resources/items you can carry, how many crafting resources goes into crafting any particular blueprint, how many and what items are created through each crafted blueprint, you can also fine tune the hacking game with the time limit (how much time on each difficulty, alarms, how many symbols you need to match etc).
You can tweak the player, Alien, Android and other NPC melee and health in the CHR_INFO/Attributes folder. There are AI variables, hearing and viewconeset info. There are settings scattered in the CHR_INFO/attributes, DIFFICULTYSETTINGS and VIEW_CONE_SETS folders tweaking vision and awareness settings.DIFFICULTYSETTINGS folder is also a good place to look into.
In the WEAPON_INFO/Ammo folder files, you can set how much, if any, damage a weapon does to the Alien, damage to other NPCs, flamethrower fuel consumption rates, how many pellets a shotgun (or other weapon) fires, what kind of damage weapons do, ranges, EMP, Stun, Blind durations etc.
Some of the XMLs do seem to hold info on graphics as well, the most immediate I notice is in the MATERIAL_DATA folder, the Materials.xml, which seems to hold some physics engine variables for objects, and decals, debris, some effects.
Anyway, that´s a start on XML & BML editing. See more at the Steam guide or jump right into modding!
Special thanks to dannyhl2!