First grab an old t-shirt. My husband has tons, but only actually wears a few. Finding some to cut up is never hard here. If you don't want to cut up one of your own, thrift stores always have a lot to choose from. Bigger ones make longer dresses or tunics (my excuse for not cutting up my own).
Then just cut off the sides and top. You can cut it in a slight A line shape if you want. Be careful to cut straight, parallel lines or the finished product looks a bit wonky, but don't stress too much about it. The t-shirt fabric stretches and warps itself anyway covering mild imperfections.
To make cute little decorative flowers, cut circles out of the sleeves. About 2 inches diameter is a good size. You can cut any shape, but circles are the easiest.
Grab one more t-shirt in a coordinating color and cut off the bottom then a 1-2 inch strip just above that. Cut the ends to make the two drawstrings for the collar/shoulders of the dress. You can also cut some 1 inch diameter circles to be the centers of the decorative flowers.
Now the sewing starts. Sew up the sides stopping about 8 inches from the top. Fold over the top (wrong sides together) 1 inch and stitch 1/4 inch from the raw edge to make a tunnel for the drawstring.
The most time consuming part of the project is stitching on the flowers. A simple X in the middle holds them in place and allows them to flop and ruffle. Arrange them any way you like.
Attach a safety pin to one end of one of the drawstrings and thread it through one tunnel. Repeat with the other one.
That's it. An ugly old t-shirt is now an adorable pillowcase dress. On my older daughter, these are tunics which look great with leggings or capri jeans. On me, they are cute little tank tops. All in all, the whole project takes less than an hour making it another quick and easy fix for sure.