You will need a icing bag, scissors, a plastic coupler and ring, and icing tips. I recommend a twelve-inch disposable icing bag because a larger bag would get in the way. Any kind of icing tips will work, but for straight lines, a #10 plain round tip is good for bolder lines and a #2 plain round tip is best for finer lines.
Couplers are a huge time saver if you are planning on using different tips for one cake. You can change tips without having to change bags.
Insert the coupler into the bag. Snip off the top.
Once the top of the icing bag has been cut off, push the coupler through so a few of the screw threads are visible and the fit is tight.
Attach the icing tip you have chosen to the icing coupler, then place the plastic coupler ring over the tip and tighten.
To keep both hands free, place the icing bag over a wide-mouth quart jar or tall glass and cuff the excess bag over the edges.
Now that one hand is free, you can hold the jar or glass with one hand and, using a spatula, fill the icing bag only 2/3 full, overfilling will make a mess.
Squeeze the filled icing bag to get out any air bubbles. Then twist the end and cradle the icing bag against your palm between the thumb and rest of the fingers, wrapping the fingers around it and squeezing with the fingers to control the flow of icing. Using your other hand to hold on to the tip for extra support.