For the collar, cut off about 5-6 inches off of the bottom of an old t-shirt and cut along one edge so you end up with a long rectangle of fabric (you can use the same amount of knit fabric off the bolt, if you prefer. No need to hem it). Don’t worry about length too much for now. As long as it goes all the way around the collar of your t-shirt, you’re fine. We will cut off the excess later.
Fold the rectangle in half lengthwise, and topstitch all along the folded edge using a wide zigzag stitch.
Open up the fabric, and fold in one edge until the collar is at the height you’re going for. If you used a t-shirt, folding at the hem stitches gives you a great, even guide. You can press it, if that’s how you roll. I didn’t.
Starting from the back of the costume t-shirt collar, pin the folded edge of the collar to the shirt all the way around. Keep the collar opened up.
Once pinned all the way around, fold down the inside of the collar so that the collar of the t-shirt is sandwiched between the front and back of the new collar. When you get to the back (where you started pinning) give yourself a few inches of slack and cut off the excess material.
Now, this is important, set your stitch to an overcasting stitch. Don’t use a regular straight stitch, because it won’t be stretchy enough when you try to pull the collar over the head, and the thread will break. Most regular sewing machines have the overcasting stitch I have circled above. If yours doesn’t have it, you can also use the stretch stitch (#3 in the picture) or a regular zigzag stitch (#4). Still not sure? Check your sewing machine manual! Have you ever read yours? There is so much crazy good sewing information in those things! A couple of other things to think about: Make sure you change your foot, if necessary. And you may want to use a ballpoint needle. I used a universal needle, and it was fine, but most sources I have read recommend a ballpoint needle for sewing knits.
Time to attach the collar! With the new collar on your left and the t-shirt on the right (you may have to maneuver a bit to get this set-up) use overcast stitching to join the new collar to the t-shirt collar. Make sure that the inside of the new collar is neatly folded down, and the t-shirt collar is sandwiched in-between.
Since I used white thread on white fabric, it was hard to take a decent picture showing what the overcasting stitch actually looks like, so here’s a picture of it where I attached a green collar piece to a white t-shirt using black thread.
Go all the way around, stopping a couple of inches before you reach your starting point. Give yourself about an inch of overlap, and cut off the excess fabric.
Use your seam ripper to remove the zigzag stitch along the top about an inch back. Slip the beginning of the collar into the end of the collar. Fold the raw edge under. Finish sewing to the beginning point along the edge of the collar. Then, with your needle still down, lift the foot and pivot the material and sew along the back seam, joining the beginning and the end of the collar together.