So first-things-first: the golf bag goes into the slots on the back of the golf cart. There is room for two bags between the wheel covers on either side of the rear of the cart.
See the black straps hanging from the plastic bar? That's one strap for each bag. Each strap wraps around a bag and is secured by the "buckle" on the end of the plastic bar (the buckle on the left side is in the open position; the one on the right is not visible because it is in the closed position).