Using scissors, trim the end of the bag to fit the coupler base or piping tip.
Insert the coupler base into the bag, pushing it down to the tip.
Place the piping tip over the coupler base and secure it with the ring.
If not using a coupler, push the tip directly into the bag and trim the bag to fit snugly.
Fold the top of the bag over the edges of the cup or glass to keep it open.
Using a spoon or spatula, fill the bag with icing or batter. Do not overfill, you want to fill about half of the bag (or less for buttercream frosting).
Remove the piping bag from the cup and squeeze the filling down towards the tip, removing any air bubbles.
Twist the top of the bag to close it securely and its ready to pipe.
To switch tips with a coupler, unscrew the ring, replace the tip, and re-secure the ring.