How do you apply weapon mods to your guns?

[76561198134248138]

I have never figured that out in 20 hours of gameplay. I tried searching on forums but it didnt helped too. What button do i need to press to bring up the weapon mod menu so i can attach them on my guns?

[Discreet_Pigeon]

Press the X key when you are under the weapons category on the pip-boy.

Edit: You must highlight weapon then press the X key which will bring up a menu to mod weapon that is highlighted only.

[Cake for Mumm-Ra]

When you're looking to buy a weapon mod from a vendor, look under the MISC tab

[max.wolf]

You need a) a moddable weapon and b) the appropriate mod both in your inventory. No workbench or anything required to attach it, simply select the weapon, press X, then another menu displaying the appropriate mods appears and you can apply them (probably by pressing E).
Mods are not generic items, i.e. each weapons mod is unique for each type of weapon. Their names also say it, actually; for example: 12.7mm Pistol Suppressor, 12.7mm SMG Laser Sight, Laser Rifle Focus Optics, Holorifle Focus Optics....