How do I modify my weapons?

So I've found several weapon mods that sound pretty awesome, like a scope for my SMG, and improvements for my Shotgun.

Rad.

And I've found a 'Weapon Bench' which, when I go to alter my loadout, also has a convenient 'modify' button that shows me two empty mod slots on all of my shiny new guns. I can inspect the 3D model of a given weapon more closely, and at multiple angles, all very nice.

But I cannot, for the life of me, figure out how to attach one of these shiny, awesome mods, to my shiny deadly guns. I see two 'slots', one of which is lit up in red as 'active', and I can switch which one of those two is selected, but I can't see the mods anywhere to add them, and I can't figure out where on the screen they would be.

How do I modify my guns?

Modifying your weapon is a bit of a convoluted process.

First, you need both a modifiable weapon, and a weapon mod. For instance, you can only stick an SMG mod on an SMG - no SMG means you can't use the mod at all.

Second, you need to find a weapon bench, or go to the armory on the Normandy and use that.

From here, simply click the appropriate weapon class, and then the red "Modify" button.

Next, you want to select one of the two mod slots. Note the counter here. If the number is 0/0, that means you have no available weapon mods (at least for this weapon). Pressing the left/right arrow will let you browse through available mods, and the up/down arrow will move between the two mod slots.

Simply manipulate these two arrows until you've got what you want, and then press "Confirm" on the lower left.

Ta da! Brand swanky new gun!