If Your State Doesn't Recognize Legal Separation
Negotiate a separation agreement with your spouse. Make sure you resolve all issues. If you have children, you'll need to address not only custody, but visitation and child support. Decide how you're going to divide both property and debts. Determine whether one of you will pay spousal support, or alimony, to the other. You should cover all the same issues you would have to address if you were divorcing instead.
Draft your agreement in legal form. You can download a form from a legal document website or you can draft your own, incorporating everything you and your spouse agreed on.
Sign your agreement in front of a notary. When you and your spouse both sign, it becomes a legally binding contract and you're officially separated. There's no need to file it with family court, unless or until you eventually want to file for divorce based on the same agreement.
If Your State Does Recognize Legal Separation
Research your state's grounds and residency requirements for filing for legal separation. These can vary from state to state.
Access a petition for legal separation that conforms to your state's laws. Many states offer family law forms on their judicial websites. If your state doesn't, call the court or download one from a reputable online legal document provider. Just make sure that what you download or purchase is intended for use in your state.
Complete and file your petition. Typically, petitions for legal separation are filed in family court in the county where either you or your spouse reside.
Serve your spouse with a copy of your petition. Different states have different rules for this, but you can usually ask your county sheriff to hand-deliver your petition to your spouse for you. After you've accomplished this, the steps toward legal separation are usually much the same as those for getting divorced. At the end of the litigation, a judge will rule on issues between you and your spouse and issue a separation judgment or decree of separation.