Alright, so you know about the Law, the Gospel, sin, salvation, the Four Spiritual Laws, and the Roman Road, but how do you lead someone to the Lord? How do you introduce someone to Christ?
First of all, you tell them about God, sin, and Jesus dying on the cross. So, let's say that after all this, the person you're witnessing to is touched by God's Word and wants to become a Christian. You say...
"Do you understand what I've been telling you?"
"Do you want to receive Jesus as your Savior?"
"Yes. Yes I do."
"Good. But first, I need to tell you something. Being a Christian means living for Christ. It means seeking to do His will and not your own. It won't always be easy. Satan will make your life difficult at times. You may lose friends and things won't necessarily get better overnight. Being a Christian can be costly. In fact, Jesus said that you must deny yourself, take up your cross daily, and follow Him (Luke 9:23). That is what Jesus wants you to do, to follow Him always -- even when others don't want you to.
"Now, If you still want Jesus as your Savior and Lord, I would be glad to pray with you."
"Yes, I do. I understand and I still want Jesus."
"Then let's pray." You tell him to repeat what you say, maybe bow his head and close his eyes, or not... It doesn't matter.
An example of what to say is "Dear Jesus, I know I have sinned against you. I confess that I am a sinner. I accept the sacrifice you made on the cross on my behalf. I ask you to come into my heart. Please cleanse me of my sin. And give me eternal life. I repent from my sins and put my trust in You. Thank you Jesus. Amen."
The preceding paragraph is only an example of how to lead someone to Christ. As I've said before, there is no formula, but only a recognition of one's sinfulness before God, confession and repentance of sin, and trusting in Jesus' sacrifice for sin.
I would like you to take special note to tell the person that being a Christian is not easy. In a way, when you do this you are trying to talk him out of being saved. But what is really happening is three things: 1) you are making sure conversion is real, 2) you are warning him of what is to come, and 3) you are trying to prevent a 'false conversion' and the excuse he might use later, like "Oh, I tried Christianity once. It didn't help me." Remember, we receive Christ for the forgiveness of sins, not to "make things better." Also, you want someone to come to Christ with full knowledge of what it means to be a Christian.
If we aren't willing to follow Jesus through thick and thin, then we aren't worthy to be called His disciples. We are to love Him more than anything else (Luke 14:26-28).
Also worth noting is the short sentences used in the prayer. Don't worry about exactly what to say, just speak the truth, confess sin, ask Jesus for forgiveness, speak slowly, and use short sentences. This way, there will be no confusion.