How do you use perks in call of duty ghosts?

Well, you don't really USE perks. They kind of just alter your overall physical performance. They don't really give you any equipment, weapons, or anything like that really. Once you've equipped them, you're good to go.

If you're asking how to equip perks, it's easy. Just select the perks section within one of your loadouts, and pick which ones you want. They're all organized into categories to make it easier to pick (ex. Perks that improve your reload speed, aiming speed, and more are found in the category "Handling"). However, since you obviously can't just pick'em all, they utilize a point system. An empty loadout (no weapons or equipment either) contains 12 unused points. Each primary weapon, secondary weapon, and lethal/tactical grenade uses 1 point. Whatever you have left (note: you don't have to actually equip a weapon or grenade) can be used to equip perks. Perks are also organized by their cost of points. The further along the row of perks, the more it costs, but the better it is in general. Once you run out of points, you're done. For example, if I equip a Sniper rifle (-1 point) and a C4 (-1 point), I'll have 10 points left over, which I'd use to equip, say, Quickdraw (Faster aiming down sights, but -3 points), Dead silence (Silent movement, but -2 points), Scavenger (Resupply from enemy bodies, but -2 points), Ready up (Raise weapons faster after sprinting, but -1 point), and Sleight of Hand (faster reload, but -2 points). These obviously aren't the only perks, so this system allows you to make a loadout that best suits your play style and allows for more customizations.

Hope this helps!