To make a plain apron, first measure and write down the measurements below.

if you’re making an apron that will be tied, extend this length by at least 1 ½ times the measurement.
decide how wide you’d like the waistband to be, adding 1 inch for the seam.
decide how far down you want the apron to fall and measure that, adding at least 2 inches for seams and the hem.
measure the width you’d like the apron to be from one side to the other, adding 1 inch for hems.
cut your selected fabric in the dimensions you’ve written down.

which step is missing from this procedure?

measure your waist and add 2 inches.
sew the apron by hand using a long running stitch.
measure the fabric before you cut so that there are no wrinkles or flaws that may need to be cut away.
invite a few guests over and wear the apron proudly as you cook a gourmet meal for them.