[1][2] A special transfer sheet, usually ISO A4 size, is printed on with a regular inkjet printer.
It is often a requirement of "homemade" garments (adhesive-based transfers) that they are washed inside out, only in cold water, sometimes by hand, and not be tumble dried.
[3] The second type of transfer paper is merely a substrate which ink is deposited onto, from which it is sublimated directly into the t-shirt fibers (requires at least 50% polyester fabric).
The main advantages of this process are permanence and lack of a rough adhesive on the surface of the fabric.
The transfer sheet is placed ink side down (usually) onto a t-shirt or fabric and ironed (without steam) onto the cloth.