First, several things you must have before summoning. Awareness of the unseen, an understanding of who you are summoning, and a good reason for summoning them. Until you have those things do not attempt a summoning lest you risk opening a gate you cannot close...
Second, one does not just "sell" their soul. Thy soul is a precious source of power and to selfishly offer it to another without even understanding what it is, that sir, is a terrible idea. While some might have a different view of what a soul is I'll explain how to sell it. Your soul is a pool of essence, your essence, and could be seen as a second "body" capable of walking the astral realms. By "selling" one's soul, you are giving the buyer the usage rights to your "private property". While you live, and thus your soul is bound to your body, you will likely serve as a battery. Depending on the agreement, perhaps you sold your body as well, you might even act as a servant from time to time. Upon death, they own your now unbound soul and can do what they please with it. However, when making a deal you must know what the other side can offer. Don't be greedy, don't trust any claims of manifesting objects (be it riches, powers, or love), the laws of the universe won't change, word your offers very carefully , and never trust the other side to not twist the deal.
The actual summoning and deal-making are a bit harder than the above sounds. Once you can maintain the connection with the entity then you can start to make the deal. Since Lucifer ain't going to just physically appear before you, you will need to be able to sense him. Once you have debated the deal there will be an exchange, a tiny piece of "you" will be taken by him and replaced with "him", thus sealing the contract and branding it onto your soul. Honestly, you shouldn't be trying to do this, mind telling me why you are trying to summon Lucifer? Its just that selling one's soul is a risky venture, and even riskier if you are inexperienced.
If you'd like to know more feel free to PM me. Not like I have anything better to do.
In all honesty, I would advise against selling/pledging your soul to untrusted or dubious entities.
Truth is, when you offer up your soul, if the entity receiving it is quite insidious, they can just snatch it away without doing anything. For example, they might agree on fulfilling your demands, then kill your right afterwards, because, technically, they have already done as you requested, even if you didn't get the time to enjoy the prize of your bargain.
Why do you want to sell your soul to Satan? Are you interested in Satanism, or do you want to obtain power?
Satan even existing is very dependent on your beliefs, many people claim to want to sell their soul, very few people can say with any conviction that they have succeeded and it is likely that many of those are lying.
There are many ways to go about getting things in life, many that are far simpler than trying to sell your soul.
What in particular did you want to sell your soul for? then maybe we could help you find another, more likely way of achieving it.
Satan is an honorific term meaning adversary. It was applied to many biblical beings with individual names through constant translations. Samael, Azazel, etc. were angels with seemingly insidious tasks such as tempting humans to test that they are worthy of going to heaven, or taking the blame for the wrong in the world, hence their names' meanings being Poison of God and Scapegoat of God respectively. Then there's the dragon from revelations which may have actually been called Satan, but again, it still roughly just means enemy. The dragon was an adversary of heaven. As for selling your soul, that just isn't a thing that happens. Spirits have nothing to offer you in this world that you couldn't obtain from them without a bargain of such magnitude. They cannot offer you wealth, or physical alterations, or anything that you might be hoping to trade it for. Not that there's anything in this finite life worth trading eternity for, if that's the sort of system you believe in.