To attach a weapon mod, a screwdriver, the appropriate weapon to modify and the weapon mod, must all be in the player's main inventory. Upon RMC the weapon, an option to "Upgrade" will appear, showing all currently available weapon upgrades. Upon LMC the desired upgrade, the player will equip both the screwdriver and the weapon mod, before attaching it to the weapon. To remove the weapon mod, simply follow the same steps, however choosing the option to "Remove Upgrade" followed by the desired upgrade to be removed.
“Those modifications will come with some luck directly mounted on the rifle, or you can fine separated parts at a gunstore.”