A variety of tubes (below) can be used with a pastry bag, some plain for shaping meringue and choux pastry, others cut in shapes for piping decorations of whipped cream and butter cream (here).
Drop a tube into the bag and twist, tucking the bag into the tube. This will prevent any filling from leaking out at the bottom.
Fold the top of the bag over your hand to form a collar and add the filling, scraping the spatula against your hand.
When the bag is full, twist the top until there is no air left in it.
Holding the twisted end of the bag taut in one hand, with the other gently press the filling at the top of the bag to start it flowing.