Step on of this project is to gather your stuff... old t-shirt, scissors, pen/pencil, plate, and sewing machine (I suppose you could hand sew)
Lay the shirt flat and naturally ( the front neck will be lower the the back of it a little bit then place the dinner plate on the shirt in the middle of it. Using a marker lightly mark around the plate you can either trace one long line or a few spots.
Then remove the plate and begin cutting the neck off. Cut just outside the line so it will go with the scrap. Once it is all cut it will look something like this.
Turn the shirt inside out and line up the bottom seam and using a straight stitch sew right in the existing seam. A straight stitch worked fine for me, but really, whatever makes you comfortable, don't sew backwards in the beginning and at the end to kind of lock it in place.