First things first: lay the sweater flat on your work surface and use
your tailor's chalk to mark 1.5" down from each armpit of your sweater.
Use a ruler to connect a straight line between these two marks.
Next, draw a horizontal line 2" above the bottom line, and a second
horizontal line, 4" above the bottom line (leaving you with three
lines, with 2" in between each line.)
Finally, find the middle of your sweatshirt, and draw a vertical line
directly up the center. This is roughly what your sweater should look
like when done.