First: draw the diagram as shown in the image.
Keep these facts in mind for a good design
1. The wing shouldn't be on the center of the fuselage but a little towards the nose.
2. The area of paper used on side 'A' should be equal to side 'B'. This can be done by first measuring the area of side 'A' and then adjusting the length of 'X' so that the area of 'B' would be the same as 'A'.