You want to cut at a diagonal, so that the straps will stay on their shoulders. So at the top of my cut, it measured an inch over from the seam. Do not go more than an inch over, since that would defeat the purpose of the square-neck:
Measure straight edge at top. Mine was 5″.
Now place the pieces you cut apart back together, and measure from the start of your cut, to the neckline, so you’ll know how long to make the straps. Mine is 6″. You’ll need this measurement for the straps.
**TIP: Your straps and strips will be more stretchy if you sew across the vertical lines on knit fabric. The pictures don’t show me doing that. I learned my lesson, though! Sachiko has a great explanation HERE. See trick 3.
You’ll have this now:
Fold strip over.
Your onesie should look like this now:
Take one of the 13″ straps. Starting at the back corner, pin the strap around the armhole.
You can basically just turn the fabric so you sew a straight line, even though it is a curved arm area.