So first we take a plain white T-shirt. We lay it flat on a whole sheet of newspaper, so that the floor does not get painted. We also fold another sheet of newspaper and slide it gently between the front and back of the T-shirt. Prevents carbon-copy-like effects on the back. Also helps if one wants to do some other artwork on the back.
Next, one decides what one wants to paint. The child was keen on the Sorting Hat, so the Sorting Hat it was.
So she drew an outline of the sorting hat on a sheet of stiff paper and cut it out. Then she placed it where she wanted it on the T-shirt and traced out the outline.
Next, one proceeds to draw the top of Harry's head just below the Hat, complete with round framed glasses, unruly hair, scar on the forehead and tightly closed eyes, worried about which House he would be sorted into. "Not Slytherin, not Slytherin", he says, as the Hat wonders where he is to be placed.
Then we proceed with the fabric paint, and fill in the picture we have just drawn. The Hat shouts out GRYFFINDOR!!!
Capturing this moment just as the Hat shouts, and before Harry reacts too it, eyes still tightly closed!