Cut your fabric pieces to the desired finished size, plus 2 inches for the hems (each hem is one inch).
Fold the raw edges over 1/4 inch on all sides and press.
Fold the edges over again 3/4 inches on all sides and press in place. See how that edge is sticking out? That is what happens if you don't fold the end of the first hem over a little as described above. Here it doesn't matter because we aren't ready to sew the hem just yet.
Gently unfold the hem at one corner. Make sure that the innermost fold line is very visible.
With the initial 1/4 hem still folded over, fold the fabric at the corner so that the folded edges meet as shown in the photo. Take note of where the second fold crease ends (where the arrow is pointing).
Take a ruler or square and line up the straight edge with the point where the crease ends. Make sure that the right edge of the ruler or square is aligned with the fold you made in the previous step. This will ensure that the line you mark is straight.
Using a marking pen/pencil or chalk, trace a line along the edge of the ruler. Adjust the ruler, if you need to so that the line is in the right place - it needs to start right at the crease point. Pin fabric on marked line to keep the corner folded correctly.
Repeat the folding and marking process on the remaining corners and pin.
Take your prepared napkin to the sewing machine and repin the seams so that the pins are out of the way or easy to remove as you sew (make sure that the hem fold edges stay aligned). Sew directly on the marked line on each corner, backstitching at the beginning and end of each seam. Trim off the point so that the seam allowance is 1/4 inch as shown in the photo.
Turn hems right side out and gently push out each corner so that they are sharp. See how nice that corner looks now?
Sew around the napkin, close to the inner folded edge.
I love the look of this mitered corner and the wider hem does not curl as much as the regular double-fold hem does, so the extra time that it takes to prepare the hem is worth it to me. Try both techniques out and see which one works best for you!