If someone leaves the church what can he or she do to return? Another question: Does a person who leaves the church lose thier salvation?
The Bible does not prescribe a fixed means by which those who have chosen to leave the church should be returned to the fellowship. We could look to 1 Corinthians 5 and later 2 Corinthians 2:5-11. These describe the disfellowship of a disciple who falls into public sin and how he (or possibly another person) is welcomed back into the fellowship. I think this might give us some helpful ideas, but the context here is quite different. This is about a person who is driven from the church for public sin. I assume you are talking about a person who chose to walk away from the fellowship. Galatians 6:1-5 may be more helpful here. Pauls urges us to try to restore people to the fellowship, but to be cautious lest we become caught up in the very sin of the one who fell.
Bottom line the Bible does not give us a specific list of things which must be done in order for a straying sheep to come back into the fold of the church. Each local church should decide for themselves how they will handle this. Some common sense Biblical principles apply. Like we see in 2 Cor 2 and Gal 6 the errant disciple should be welcomed back with love. The parable of the Prodigal Son surely applies here. In that case, what was required in order for the son to return to his Father’s house was simply for him to decide to get up and come home. I do not think we should put up large barriers to prevent people from coming back. If the evidence is strong that someone has truly repented and wants to return to the family of God, then we ought to receive them with open arms.
Of course this does not apply to all cases. Some want to sneak back into the fellowship under the radar. They want to come back but are not fully ready to repent and be restored to a truly committed Christian lifestyle. For this reason, I believe it is wise on the part of the leadership of a local church to have some sort of policy–be it formal or informal–of interviewing those who come back to make sure they really are ready to fully engage in the church and to submit to the leadership of the church. If they are not ready, I believe something like a series of studies may be advisable or possibly the church may choose to hold off for a time on receiving the person into full fellowship with the church. No single policy will cover all cases.
Does one lose one’s salvation when one leaves the church? I think that it is possible to lose our salvation by continuing willfully in sin over a long period of time. This is what is taught in Hebrews 10:26-35 and Hebrews 6:4-8. This can happen to those who are still in the fellowship or those who have left, but of course those who leave are far more likely to fall into this terrible condition. We, as human beings, are not able to discern who has "fallen away" and lost their salvation. This is God’s territory, not ours. Our job is to try to bring the wandering disciples back into the fold. If they are willing to be renewed to repentance, then we know that they have not lost their salvation and we should receive them again into full fellowship with love and open arms.