Once you have setup the proper dimensions of your drawing you will need to place a marker to bisect the canvas along the vertical axis. This marker will define the front and back regions of the bottle. Use the left side to disign the front label and the right side as the smaller secondary label if you want one. Remember that the edges of the canvas at the left and right sides will touch each other when etched onto the bottle. This allows you create a continuous design that will wrap around the entire bottle. I recommend placing a repeating patter about 1/2" from the bottom of the bottle as it adds a nice touch to the design.
When designing the front label keep the design centered with respect the left segmented region of the canvas. Follow the same practice with designing the back label. This will place the labels on the exact opposite side of the bottle. I also like to place the back label near the bottom of the bottle which give it a well designed appearance.