Step 2: Make the Pattern
You're going to end up with two pattern pieces -- one will be the apron, one will be for the casing. Start with the apron. Measure out the dimensions onto the paper and then draw in a nice curve for the armhole.
Now you need to make the pattern for the casing. Measure in two inches from the arm curve and mark it with dots several places along the way. Connect the dots so you've got a consistent two inches all the way along the curve. This is the shape of the casing piece you're going to need for the tie to slide through. If you're using parchment paper or other translucent paper for your pattern, place another sheet over your pattern and trace the curved boomerang shape. You can tape it to a big window or a sliding door during the day to make the lines more visible.
Now cut out the pattern pieces. You should have two pieces of pattern paper now, one apron piece and one casing piece. Make sure when you cut the apron out that you cut along the very outside edge. Do not cut off the piece with the casing drawn on it.
NOTE: If you really don't have a single piece of paper you can see through, using wrapping paper or other paper you can't see through, slip the second piece under the apron pattern, trace around the apron pattern, and the measure two inches in on the casing pattern piece. Make sure you cut along the OUTSIDE edge of the apron pattern.