How do you add the weapon mods to a weapon?

Starting with a search method.

RUDE ! ! !

People come here for help not a [censored].

If you are on PC chose your weapon and then press x.

Equip the weapon. Choose the option to mod. It'll take you to a list of the weapon mods you have. Highlight the mod you want and activate. The weapon will be modded.

Awesome! Thanks! I did try searching the forum to begin with but nothing came up.

You don't have to equip, just click weapon in pip boy. Under repair option should be modify option. You must have parts in your inventory else its greyed out.