Measure the big PET bottle around. Write it down.



Draw a line a little bit above the middle of the bottle so you have so you have a cutting guide.



Cut the bottle in half with the utility knife. Warning: PET is really easy to cut with an utility knife, so if you have never cut a PET bottle before, then you should know that making a straight line is almost impossible. So make a little cut with the utility knife and finish cutting with scissors.



Now, measure the neck of the bottle. If your bottle has one of those plastic handles to lift it, just cut it out with the utility knife.



Divide the length of the bottle neck between the number of panels that your lamp shade design has.

Then, divide the length of the bottle (the one you wrote down before) between the same number of parts.



With the permanent marker, mark the neck bottle as needed, using the number obtained before; but make sure the first mark is drawn over the PET bottle's union line (the line that runs from top to bottom of the bottle dividing it in 2). You will use this line as a reference and as your first vertical line.



If you are having trouble getting your neck bottle's measures OK, specially placing the division marks, place a piece of masking tape all around the neck bottle right in the place where the neck bottle starts to get wider. Cut the tape and remove it from the bottle. Then stick it in the cutting board, measure it, mark it and then cut it by the markings to get little pieces. Stick the little pieces back into the bottle and then make the marks.



Mark the wider part of the PET bottle starting at the union line.



Stick a length of masking tape from a mark in the neck to it's corresponding mark in the wider section of the bottle. Use it as a guide to draw a straight line. Repeat this with all your markings.



My lamp shade has 6 panels (see the last photo).