The essence of mindfulness practice can be found in a text written a couple of millennia ago called the Satipatthana Sutta or the Discourse on the Four Foundations of Mindfulness (from the Majjhima Nikaya, The Middle Length Discourses, MN10). It covers the Buddha's comprehensive, practical instructions on the development of mindfulness. I have found reading the actual text, even in accessible English translations like this one by Nyanasatta Thera and this other from Thanissaro Bhikkhu to be only somewhat useful. Far better for me is listening to talks on individual exercises like those linked to below.
Gil Fronsdal has given a series of talks on the Four Foundations of Mindfulness several times @ the Insight Meditation Center in Redwood City, California. Below are the links for the 2016 talks. See also this page on the audiodharma.org site for a similar series from 2019 and another from 2003.
Click "Download" to download an MP3 of the talk to your device. Click "Stream" to listen through your browser. Each of these is 40 to 45 minutes.
2016
2016