If you have not figured it out yet its really easy. Go to the gun you want and click triangle on it. This should bring up your skins. Then click on the left one button and click over till you c your clan tag box and emblem box. You need to click on the clan tag box so it high lights. Once you did that, your golden. Any more questions just ask.
You do understand that your gun needs to be prestiged once for clan tag and twice for emblem before you can add your clan tag or emblem. FYI