Make a solution of 2 parts water, 1 part paint (or use leftover brush-soaking water with paint). Lay out the shirt over scrap paper and drip 1 tsp. of the paint water on the cloth. Wait a few seconds, then check to see if the paint soaked through. If it did, don't use this shirt for a smock unless you double it up with another shirt, because it's too thin to protect clothing beneath.
Trim the sleeves of the shirt to a length that will leave the wearer's arms free. Leave them just long enough to cover most short-sleeve fashions.
Remove any pockets from the front of the shirt using a seam ripper. Work carefully to avoid damaging the fabric. Set the pockets aside. If the shirt has no pockets, cut out two or three rectangular pieces of scrap fabric measuring 6 by 4 inches.
Use the seam ripper to remove the shirt's buttons as well. Discard these or use them for another project.
Sew the pocket pieces in place on the back of the shirt (which will be the front of the smock). Position them at least 10 inches below the collar, but otherwise, place them wherever you like. Sew by hand or with a sewing machine using a running (straight) stitch on all but the top side of each pocket.
Attach strips of self-adhesive Velcro to the button-down area of the shirt to replace the buttons. Place the rough side of the Velcro strips on the underside of the strip that had the buttons, facing down, and place the soft side of the Velcro on the side with the button holes, facing up. This will be an easier way for the wearer to put on and remove the smock from behind.
Put on the smock (or put it on the person who will wear it). Tie the sash piece around the waist until it fits comfortably. Tie it in the front or back.
Pin the sash in place at the sides with safety pins.