By St. Symeon the New Theologian
Above all else you should strive to acquire three things, and so begin to attain what you seek. The first is freedom from anxiety with respect to everything, whether reasonable or senseless - in other words, you should be dead to everything. Secondly, you should strive to preserve a pure conscience, so that it has nothing to reproach you with. Thirdly, you should be completely detached, so that your thoughts incline towards nothing worldly, not even your own body.
Then sit down in a quiet cell, in a corner by yourself, and do what I tell you. Close the door, and withdraw your intellect from everything worthless and transient. Rest your beard on your chest, and focus your physical gaze, together with the whole of your intellect, upon the center of your belly or your navel. Restrain the drawing-m of breath through your nostrils, so as not to breathe easily, and search inside yourself with your intellect so as to find the place of the heart, where all the powers of the soul reside. To start with you will find there darkness and an impenetrable density. Later, when you persist and practice this task day and night, you will find, as though miraculously, an unceasing joy. For as soon as the intellect attains the place of the heart, at once it sees things of which it previously knew nothing. It sees the open space within the heart and it beholds itself entirely luminous and full of discrimination. From then on, from whatever side a distractive thought may appear, before it has come to completion and assumed a form, the intellect immediately drives it away and destroys it with the invocation of Jesus Christ. From this point onwards the intellect begins to be full of rancor against the demons and, rousing its natural anger against its noetic enemies, it pursues them and strikes them down. The rest you will learn for yourself, with God's help, by keeping guard over your intellect and by retaining Jesus in your heart. As the saying goes, "Sit in your cell and it will teach you everything."