Iron-on sheets have come a LONG way... Two different options; one for light colored shirts and the other for dark fabrics.
Take the drawing, scan it into your PC, lay a sheet of 8.5x11 paper on the shirt to get an idea of the image size you'll be dealing with, and then use the 'print-preview' function to gauge how much of the actual transfer paper will be covered by the image.
Print a few test copies on regular paper prior to printing on the transfer paper.
We've done a number of shirts for my son whenever he needed something for school (The Lorax Book report), or a themed birthday party (Adventure Time.. Schmowzow!), etc.