Summary: You cannot achieve a good conscience unless it is lead by God.
How to achieve a God Lead Conscience
Romans 2:15 (quickview)  Which show the work of the law written in their hearts, their conscience also bearing witness, and their thoughts the mean while accusing or else excusing one another;)
Conscience is defined as:
An inner witness to God and righteousness
An instinctive knowledge of right and wrong
Thoughts – accusing or approving
Many people think that your conscience is the Holy Spirit, but this is not the case. Verse 15 teaches us that God has placed His laws within the heart of every human, without regard for their knowledge and faith in God or lack of it. We all have an inborn sense of right and wrong and this is our conscience. When we become saved and filled with the Holy Spirit, our conscience is a tool that the Holy Spirit uses to convict us.
Before we look at what a builds and constitutes good conscience we must first look at the causes of bad or evil conscience. What is an evil conscience, a simple but true answer is that it is the opposite of a good conscience. A good conscience is one that is effective, it convicts us when we have done wrong, and it encourages us when we do right. An evil conscience encouraging also. When a person with an evil conscience, they are encouraged by that evil conscience when they do wrong. An evil conscience will tell you things like, “Her marriage was on the rocks anyway, that store has a lot of merchandise they won’t even miss what you just stole or pornography is alright if you do it in your own house. We are talking about an evil conscience, Just rooting you on as you continue to sin.
This is an excellent description of a person with an evil conscience. So an evil conscience is one that substitutes that which is wrong for that which is right. The standard by which an evil conscience judges I faulty and the result is a conscience that fails to work the way God planned for it to work.
Now what causes this evil conscience. First of all it stems from an improper concept of God and of sin. People take lightly the Word of God, especially that part that condems sinful activity. This results in not believing that the wages of sin is death. As a person begins to think improperly about God and sin, it is just a short step from there to putting those improper thoughts into action.