Mark a line three inches (75 mm) from the base of the bottle. If you leave the label on you can use it as a cutting guide. If your bottles are identical you don't even need to mark - just follow the label around. Draw down to the ridge around the indentations about 1" (25mm) up from the base. Do this in the 4 places.
Cut around the bottle, and then down to the bottom ridge. Then cut across in two places to give the shape shown in the third picture.