You can absolutely make a pillowcase dress from an old pillowcase. I have seen some beautiful ones made from vintage lace pillowcases. However, I find it easiest to make them from purchased fabric.
Step One
Cut out your pieces using the size chart above. The length is the length of the dress from shoulder down plus the 2-3″ band at the bottom of the dress.
Step Two
Next, you need to cut your armholes. Since I made a size 3-6 months I measured in 2″ from the sides of my dress front and back pieces (along the 21″ side) and 4″ down (again refer to the chart).
Step Three
Turn the dress wrong side out and stitch down the side seam with a ¼″ seam allowance. This will catch the raw edge of the seam and leave you with a nice finish on the inside of the dress.