We have gone through a lot of trouble to make the process of adding new Gene Mods simple and flexible. It is done almost entirely by editing one configuration file:


First, make the mod create a Gene Mod Template for you by adding a new name to the list:
This name can be anything, just make sure it is not already used by some other Gene Mod.


Second, configure your new Gene Mod Template. Start by adding a configuration header:
Anything you type below the header will affect only this specific Gene Mod Template.

Here are the things you can configure:

Gene Mod Image
This image is used only in various popups that have to do with this Gene Mod.

Gene Mod Ability
Each Gene Mod must add an ability to the soldier. This is necessary, because the mod will use that ability to check if the soldier has this Gene Mod or not. Some ground rules:

• You CAN specify an ability added by another XCOM mod.
  
• You CANNOT specify an ability added by another Gene Mod.
  
• If you don't specify an ability, the mod will create a dummy passive ability for you automatically and assign it to the Gene Mod. Its template name will be: Your_GeneMod_TemplateName_GMPassive This makes it possible to create Gene Mods that only apply passive stat changes.
  
• The ability must be able to work without being assigned to a particular inventory slot. I.e. abilities that are normally attached to weapons, such as Rapid Fire or Chain Shot, will not work as Gene Mods.
  
• Currently it is not possible to make one Gene Mod add several abilities at the same time. The only way to accomplish it is to add them as AdditionalAbilities in the gene mod's Ability Template.

Ability Image
The icon image of the ability added by the Gene Mod. It is used ONLY by automatically created passive abilities. If you don't specify your own icon image, the passive ability will use the default Gene Mod icon image.

Time to completion
This is how long it takes to add the Gene Mod to the soldier, in days.

Stat Changes
Each Gene Mod can add any number of permanent changes to soldier's stats. The only way to reverse these stat changes is to remove the Gene Mod from the soldier.
Valid stats are:
This will reduce the detection range for the soldier by 20%.

Gene Mod Category
Category affects which Gene Mods and Augmentations will be mutually exclusive with this Gene Mod.
Valid categories are:If you don't specify a category, the mod will default to "GMCat_none" - the "MISC" category. A soldier can have any number of Gene Mods in this category.

Costs
This specifies the resources that must be paid in order to add this Gene Mod to a soldier.
Any item template can be used as a resource. You can find some examples of template names here[commands.gg] or here[consolecommand.org].

Requirements
These are strategic requirements that must be fulfilled before the Gene Mod becomes available in-game.
Some tips on setting up Requirements are already included in the Gene Mods' XComGameData.ini, so I will not copy them here.
Note that the RequiredItems array just specifies the items you need to have in order to unlock this Gene Mod, but these items will not be consumed unless you also specify them as ResourceCosts.

Allowed Soldier Classes
Which soldier classes can use this Gene Mod. If not specified, this Gene Mod will be available to all soldier classes, except those blacklisted as DisallowedClasses in the mod's other XComGameData.ini.

Required Gene Mod
Lets you specify the Gene Mod the soldier must already have before they can add this Gene Mod. Useful if you want to build multi-stage Gene Mods.
THIS FUNCTIONALITY IS CURRENTLY NOT IMPLEMENTED YET.

Restrict this Gene Mod if another is installed
Lets you specify a list of Gene Mods. If the soldier already has any of the Gene Mods on the list, then they will not be able to add this Gene Mod.


You can potentially specify different configuration parameters for your Gene Mod for different difficulties.
If the Gene Mod has no separate configuration for a particular difficulty, the game will default to the configuration below the [Your_GeneMod_TemplateName X2GeneModTemplate].
Normally you would only specify different Costs for different difficulties, but you're generally free to configure any aspect of it. For example, you could make your Gene Mod provide smaller Stat Bonuses on higher difficulties, or stronger stat penalties.

This refers to in-game description of your Gene Mod.
Some ground rules:

• If you don't specify a separate localization for your Gene Mod, it will automatically use the in-game description of the ability added by this Gene Mod.
  
• If you let the mod automatically create a passive ability for you, and you don't specify any separate localization for it, that ability will automatically use the Gene Mod's description.

Gene Mod Localization
It is done in this file:
Example:

Gene Mod Passive Ability Localization
If you let the Gene Mod automatically create a passive ability for you, you can specify separate localization for it in this file:
Example: