I've yet to reach this through God's help, but from what I've read, it means a complete and total recollection of our thoughts. All our thoughts are firmly regathered together; there are no stray thoughts or a wandering mind. Like the Gadarene demoniac after Jesus had exorcised him, we sit at the feet of God in our right minds.Give me an example of what that would look like?
Next, there are no sins blotting us, or our perception, or our souls or bodies. Sin is darkness. If we are in sin, then we are in the darkness of not knowing God. As it says in John's Gospel, everyone who practicing evil hates the light and does not come to it, lest their deeds be exposed. We must repent of our sins and appeal to God Who can cleanse our hearts. For us Orthodox Christians, we have the sacrament of confession to absolve us of our sins, and the Eucharist brings Christ into our bodies in the most intimate of ways. Through repentance and the Sacraments, we are helped by God to walk from the darkness of sin and death and into God's Kingdom, to the Father of lights, so that the light of Christ may shine within us and dispel all the darkness of sin that is within us. Repentance and subduing the passions is very important, for by cleaning up the image of God within us that is stained by sin, we wipe the mud of sin away from our eyes and remove the plank from our eye, allowing us to see God much more clearly. When we have cleaned up our house and made ready for God, then we can open the door to Him, for He stands knocking, even now.
After that, we must be vigilant, watchful, to resist all temptation and to evade all snares of the devil, and also to guard against distractions of all kinds, whether from within or without. We must also be vigilant to wait on the Lord, like the five wise bridesmaids.
Then we must be humble, and practice the virtues. We cannot very well expect to find God within ourselves if we cannot see Him in others; if we neglect the least of our brethren, then we have neglected Christ Who is within them. If we neglect Christ in others, then we surely neglect Christ within ourselves, for the Christ in others is the same Christ within us! Humility is chief among the virtues; it helps us to better fear God, respect our brethren and care for them, to be patient and longsuffering with others, to be wise, to be prudent, to be chaste, and to be honest. All of these virtues are important to having a good relationship with God, which helps us to commune with Him more fully.
We must also read Scripture and the Fathers; we must be acquainted with God's Word to recognize with Whom it is that we come into communion, and what He wants of us. We must read the Fathers to have the experience of many others who have had this deep relationship with God, and to know how to interpret the Scripture.
And, to pray deeply, to have prayer of the heart, to pray in a hesychastic manner, we must begin prayer itself. We must pray for the strength and God's help in doing all that I have outlined above. We must pray persistently and vigorously, like the woman pounding at the judge's door in Luke 18. We must violently take the kingdom of heaven by force as in Matthew 11, and our weapon for this is prayer. Ask, and you will receive. Seek, and you will find.
When God finally grants you to commune with Him, the experience might be different depending on who you are. I think some Fathers talk about an unspeakable joy of finding God, never wanting to leave this contact with Him. Some speak of resting in Him. Others have a river of tears for their own sins and for those of the world. Many are overwhelmed by the greatness of God. Many are henceforth blessed with unceasing prayer and awareness of God. No one wants to leave that state.