Get you some parchment paper or a old pillow case or sheet to lay over your shirt and you can use a iron. Go slow but keep moving so you don't scorch the paint. I think of the design in sections and iron each section for about 3 to 4 minutes. It will wash out over time but you can prolong the design by putting a good soaking spray with transparent base before you iron it or pick up some SAFE TEE from coastairbrush.
Also tell your client not to machine dry if they can remember, the heat from the dryer is what will fade it. Heat brings the color out.