First and foremost, you'll need an image that you'd like to transfer.
When considering an image, keep in mind that light colors may not produce results like solid, dark colors. You'll be printing the iron on transfer with your home printer... and printers don't actually print white ink, they just assume the background color of your paper is white and ignore white ink. So if your image has white in it... the iron on transfer will appear clear in those areas, showing the color of the fabric instead of white. Very light colors may appear transparent making for a muddy result. It's best to stick with solid, dark colors that will be rich with ink and really show the contrast of your image against the fabric.
If you plan to print your image on a dark colored shirt or dark fabric, you'll need special iron on transfer paper with a white background instead of a clear background, allowing for white and light colors but with other restrictions (more on that later!).
If you're interested in selling your custom shirt or product, you will need to make sure that you have the rights to sell the image on merchandise, so don't just copy an image off of google! You can find inexpensive, high quality artwork in a variety of different subjects on etsy and other websites and legally sell shirts and other products using the artwork.
Visit VintageRetroAntique on etsy.com
for thousands of printable vintage graphics like the cute bunny used in the example.
Next we'll look at different types of iron on transfer paper.