First off, if you are not sure what it means to configure .ini files and how to best do it, I highly recommend this guide by TeslaRage

The config file you'll want to override for this mod is XComTruePrimaryPIstolsBonus.ini.

For example, suppose you want the primary pistols bonus to only grant the following bonuses:

• Mobility +3
  
• Aim +15
  
• Will -5

Then you would need the following lines in your .ini:

In this hypothetical scenario, you'll need to set bAFFECT_DETECTIONMODIFIER and bAFFECT_DAMAGE to false because the default configuration of this mod has those two stats (as well as mobility) enabled.

For all other supported stats, since the default configuration of bAFFECT_XXXX is false, you only need to set the bAFFECT_XXXX value to true when you want it and configure the PRIMARY_PISTOL_BONUS_XXXX value accordingly.