Get a rough measurement of your neck and arm holes. It doesn’t have to be perfect, just be generous with your measurement so you know you will have enough.
Cut a 1.25″ x however long your arm holes/neck whole was. By cutting a 1.25″ wide stripe I will end up with a 3/8″ band. You can cut it however wide you want, if you cut it too big, you may not leave enough room for your kiddo’s head to go through. So beware of how big you make it, depending on how stretchy your fabric is. Now iron your strip wrong sides together lengthwise.
Start sewing using a 1/4″ seam allowance.
You can either simply trim your seam allowance, or serge for a finishing touch. Trim any extra of the band length off.
Now we will attach the neck hole. Lay your band with long raw edge lining up with raw edge of neck opening at the center back. Start sewing about 1 1/2″ from short edge of band. Sew with a 1/4″ seam allowance, again gently pulling and curving at the neckline curves.
Stop sewing about 1 1/2″ away from where the two ends would meet up. You may have more length than you need. That is ok.
Measure from the short edge of the starting point 3/8″ and make a small mark. Then over lap the two ends and cut off the extra length where the second band touches the mark you made on the first band.
It will look like this. ^^^ When you lay them together the two bands should overlap.
Unfold both bands.
Lay them right sides together.
Stitch in place using the 1/4″ seam allowance.
Once sewed together, refold the band and finish stitching the band in place.
Again you can either trim the seam allowance, or serge it. Knits won’t fray so serging isn’t necessary, I just like the professional look it gives.