I believe there are other ways which soul ties are formed, but here are a few for purpose of discussion.
When a person has pre-marital sexual relations with another person, an ungodly soul tie is then formed. This can go a long way to fight future progress and relationship or marriage.
1 Corinthians 6:16-20 (NKJV)
16 Or do you not know that he who is joined to a harlot is one body with her? For “the two,” He says, “shall become one flesh.”
This soul tie fragments the soul, and is destructive. People who have promiscuous past tend to have difficulty in their future relationships. It is as though an ungodly spirit fights them.
(1 Samuel 18:1, “And it came to pass, when he had made an end of speaking unto Saul, that the soul of Jonathan was knit with the soul of David, and Jonathan loved him as his own soul.”),
Bad soul ties can also form from bad relationships.
Vows, commitments and agreements:
Numbers 30:2-5 (NKJV)
Ephesians 5:31 (NKJV)
Proverbs 17:20 (NKJV)
Gift for love, rings, jewelries, letters, flowers, clothing, etc: These are things given ones we love and care about. This is why even after five or ten years of marriage people still keep these items to remember their old or past loves. Keeping such things or memories can result in conflict and disrupt present relationships and marriage.
I have heard people say they can’t stop this act. One thing we must understand here is that sex is meant for husband and wife. Anything out of that context is a sin. These acts create platforms for ungodly spirits to enslave and oppress people.