I appreciate the order that you listed the points. I believe that we do have a great responsibility to help a fallen person regain their faith in Christ. As we all know, however, many people do no take criticism lightly, so trying to help somebody recognize their sin can often be a hard thing to do. My first response would be to use scripture in abundance. If the person believes in the Bible, then he/she should respond to a scripture that might show that something he is doing is indeed a sin.
In response to the "right spirit" question, I do not think this is an option. It is absolutely necessary to have the Spirit with you when you are trying to help someone. The Spirit, and only the Spirit, can testify of the truth (John 15:26, John 16:13, 1 Jn. 5:6). If the Spirit of God is not present, then the message of truth may not be conveyed, no matter how strong of a speaker one may be.
The balance between firmness and gentleness is quite a hard one, but I believe that if you are truly working through the Spirit, then you will be guided in your approach. Pray for the Spirit to be with you before you confront someone, and pray for the right words. Also pray for the other person to be touched by the Spirit, that he may receive an understanding of the truth.
Within the LDS belief, and I'm sure this is shared with other beliefs as well, we have some general steps to repentance.
1. Recognize that you have sinned and done wrong.
2. Confess your sins to God.
3. Feel sincere and godly sorrow for your sins.
4. Ask God to forgive you.
5. Do all you can to correct the problems your sin may have caused.
6. Turn away from your sins and do not repeat them.
I'm not sure if I should have provided with numbers with the list, since it shouldn't really be a check list.
It is ultimately God's job to forgive, but often our friends need help on their path. I'd encourage all people to never leave a person knowingly in sin. Lastly, I think it is worth it to point out an often left out step. It is absolutely necessary to forgive yourself of sins you have done. If repentance causes God to remember your sins no more, then why in eternity would it make sense for you to remember them? It is required of us to forgive all men, and that certainly includes ourselves. I do acknowledge that some people may say that we must remember them so that we do not repeat them, and I certainly understand this, but this does not mean that we remember it in a way that would hold us back or make us feel guilty. Repentance is essentially a miraculous eraser that can erase our errors, and it is always important to remember that.
I hope that in the length of this post that I have not caused confusion with my words.