Ask children to have their parents find a plain white t-shirt or other cotton item that they can use for the iron-on.
Pick up iron-on ink-jet transfer paper at your local craft or office supply store. Choose opaque transfer paper for dark-colored items and transparent transfer paper for light-colored items. Transparent transfer paper may help avoid white outlines around artwork. Always read the instructions that come with the transfer paper.
Download the free artwork.
Use an inkjet printer to print the downloaded artwork onto the transfer paper. The artwork comes as a two-page PDF document. Page one is a flipped, mirror image (backwards) of the design. Page two is a regular (straightforward) non-flipped image.
Use an ordinary iron to transfer the design onto the item. Use caution, the iron will be very hot.