Want a safe, convenient, simple, and handy biofeedback routine?
Learning to influence fingertip temperature is biofeedback "trick" many start with because temperature is an easy and cheap parameter to collect. You may own or can buy inexpensive devices, from digital thermometers to liquid crystal "dots" that allow you to explore hand and finger temperature control. Relaxation and the techniques leading to success are not easy and simple and evolve with practice.
The practice, use and even building of the other devices will appear later, but the simple technique of placing your fingertips on your neck and comparing the relative temperature of neck and fingertips is the most powerful "Wow" experience of biofeedback available.
Try it, study it, use it.