There is 10 ribs around the bottle. 
Cut 2 windows of 4 ribs each. 
From the point where the glue line will stop connecting to the pressure bottle. 
(the little round bit like a cup at the top of the picture.  
Do try the skirt on the main body, to find out where it is before cutting) 
To the point where the ribs stop at the base of the label.
(The smooth Cylindrical part of the bottle. Which will become the skirt fin) 
This will leave you with two ribs opposite to each other. 
In effect..The "struts" retaining the skirt fin in position.