You decrease the same way you do with any other needles. The method of decrease remains the same regardless of whether you use two circs, magic loop, or DPN.
So...the question is do you need to learn how to use two circulars OR do the decreases on a hat? To help with the former go to the videos and look under advanced techniques. That will teach you how to use two circs. To help with the latter we need to know how many stitches you have cast on and have on your needles now.