Flys are used for doing chest. If you use a dumbell you lay on a bench and bring the weights out to your side and back up keeping your elbows slightly bent like you give a big hug. Don't bring the weight so far down that your trashing your shoulders, make the chest do all the movement. You can do this standing with cables or there are machines where you either grab a handle or rest your forearms on and do the same movement. Unless using for warming up they should be performed last.
The Fool say in his heart "There is no God"