To create the tie, use cotton twill or similar in your desired width and cut to size. OR if you have a larger width of cotton, like the picture, fold it in half and press a seam, then sew a straight line running the length of the tie to secure open edges. Our tie is 72 inches, but you can go as long or short as you want. You'll need more inches if you want to wrap the tie around multiple times, which is why we went with 72.
Then fold over each of the two ends of the tie to finish those edges with the sewing machine. A zigzag stitch is recommended for the ends to help prevent it from fraying.
Measure the halfway point of the cotton twill length from step 1 and the halfway point of the placemat and pin the two in place there as the first guide. Pin the cotton twill on top of the front of the placemat. Then pin the remaining cotton twill that touches the placemat as well.
Then sew across the existing stitch line on the cotton twill tie, in the same color thread, all the way across. Backstitch at beginning and end.
Then, to finish things, do a vertical zigzag stitch at the end of each side of the apron, on the tie, as shown. Backstitch at beginning and end.
Another option, would be to skip steps 2-4 and punch large holes into the placemat (one on each top corner) and attach grommets. Then, just weave the cotton twill tie though the grommets and tie. It's a totally different look and just as easy, if you want to go that route instead.