Whenever using any heat transfer material, it is HIGHLY recommended that you cover the material with a PTFE* sheet when appling heat. The PTFE sheet protects the garment, the heat tansfer material, and the iron or heat press.
*PTFE is usually called Teflon. However, "Teflon" is a registered trade name by Chemours. Just like facial tissue is usually called "Kleenex". Kleenex is a registered trade name by Kimberly-Clark.