Summarize:

Make sure that the side you want to be printed on is facing up. Most transfers are translucent, so it would be best to use a white, solid-colored shirt. If your transfer is labeled as "opaque" or suitable for colored shirts, you can use any color of shirt you want. Make sure that the transfer paper is inside the pressing area, including the silicone pad and heat pad. If you are pressing an embroidered applique, set it down embroidered-side-up, adhesive-side-down. If you are pressing heat transfer vinyl, set it matte-side down, shiny-side-up. Some types of heat presses already have a protective silicone pad. If yours does not, or if you are pressing an applique or heat transfer vinyl, you will need to lay down a thin cloth on top of your shirt and transfer. This will protect the transfer. A handkerchief, tea towel, or thin piece of cotton fabric will work great for this. Pull down on the handle, clamping the press shut. You may need to adjust the pressure, especially if you are pressing an embroidered applique or patch. The handle will most likely lock into place, depending on the specific model you have. When the timer goes off, press the start button again. This will stop the press and reset the timer. Peel the paper off of the fabric while the paper is still hot. The image should now be printed onto the shirt. If you need to press another design onto the back of your shirt, insert a piece of cardboard inside the shirt first. Adjust the plate height so that there is less pressure, then press the shirt again. This way, you won't accidentally reheat the first image.
Place your t-shirt onto the plate. Set the transfer face-down on top of the shirt. Cover the transfer, if necessary. Close the press. Press the start button. Open the press and remove the shirt. Press more shirts, if desired.