The ritual is opened in the traditional way if there are multiple persons present. See your copy of "The Satanic Bible" by Anton LaVey (1969) for the basics. It's not easy, and the ritual takes a few hours. You will need a cold room that has not received sunlight for three days, and you'll need a parchment of plain paper that also has been in total darkness for three days.
You'll also need a pentagram drawn on the floor, in order to protect yourself, and you must stand inside it from beginning to end. Treading outside of it will make any mistake permanent, and you are likely to make at least some mistakes.
Keep the air heavy with incense (whichever ones you like most), and do the ritual alone at first, otherwise your concentration will stray (and you may get embarrassed). Taking a vial of goat's blood (NOT sheep's blood, ever!), scatter drops within the pentagram, but not outside it, and not on your feet. After the scattering you must not tread on the blood, otherwise you will carry it with you outside the pentagram when you leave.
Etc etc etc, blah blah blah, bullshit bullshit bullshit. The previous three paragraphs are what I call my "dark sense of humour". This is the answer that Christian Devil Worshippers enjoy. It is b-movie media crap combined with Medieval Christian paranoia about non-Christian practices. Doing the above is not only stupid, it is pathetic... unless you make money from it, in which case use ketchup and tell them it's congealed blood.
On a similar note also see the Ritual to ensure cooperation, devised by Magister Michael Rose of the Church of Satan. The ritual will ensure those who would otherwise shun you cooperate with your simple, legal request. Visit the Church of Satan website for the rest of his essays under theory/practice.
The next two answers are not taking-the-piss answers to the oft asked question, "how can I sell my soul to the devil?...".
The person who asks "How do I sell my soul" is most likely new to the Satanism - and probably new to science, new to critical thinking, new to adulthood, and generally just newy. They are most likely asking this question to the first Satanist that they have encountered. Satanism does not, however, recognize Satan as an existent being. As a result, the answer of a typical Satanist to this question could be: "Piss off you immature Devil Worshipper. Get a life and a job. Stop harassing us until you've grown some brain cells", etc.
More informatively you could quote from The Satanic Bible:
“To become a Satanist, it is unnecessary to sell your soul to the Devil or make a pact with Satan. This threat was devised by Christianity to terrorize people so they would not stray from the fold.”
It's easy. You have obviously been highly influenced by the Judeo-Christian image of Satan, so the following method will probably appeal to you... first of all: how was evil introduced into the world? It was not by Satan, who can only come by invitation and who is not the creator. Firstly, the possibility of evil was created by god. It also created human beings that we would be inherent, constant sinners. So it wasn't long before the very first human beings accepted Satan by eating the apple. Remember that old Biblical story of Adam and Eve?
The tree they ate from was the tree of knowledge of good and evil: the primary character trait that God did not want Humans to have was moral knowledge. Hence the first sin of Satanism - which is stupidity. Knowledge (if you are interested in rejecting God this means theological knowledge) is the key to becoming a proper vessel of Satanic objectives. It's not an easy path, nor a particularly satisfying one, but the road you are seeking will lead you to the serpent's wish, that we beat God through our own knowledge of just how non-existent he is. How?
The answer lies in knowing and exploiting the fundamental flaw of theism: It is not based on truth. Therefore our greatest weapon is the truth. The knowledge of science and theology is all that is required to destroy the myths of divinity.
Enough of the silly head-bashing, and back to basic science. Souls do not exist. The whole idea of 'souls' doesn't make any sense; the extent to which our personality, character and consciousness is affected by changes to our body chemistry means that it is clear to biologists and psychologists that we are our bodies, particularly our brains. It is our bodies that we should care for. Despite the evidence for a purely physical understanding of life, many still like believing in souls. So, this page hopefully provides some useful links for misinformed spiritual adventurers.
You can't sell your soul to Satan, because Satan doesn't exist, and souls don't exist. So, be productive. Make money. Find happiness. Look after yourself. Achieve real-life success... do not spend counterproductive time on your enemies, spend time on yourself. Make sure material knowledge and success is primary, along with forming good friendships. Live life holistically and carnally. Make Satan proud through living your life with intelligence and godlessness. Make yourself efficient, strong. Be self loving, do not neglect yourself. You will find you have no need for a soul, that actually, you've already sold it to Satan when you stood on the intellectual route to self-manifestation. Now all you have to do is work out what kind of Satanist you are!