You can do a separation agreement without an attorney but I would STRONGLY suggest that you have any such agreement reviewed by attorney before signing.
Little things that seem OK and harmless can have big ramifications over the long haul that are not readily apparent and some statements placed in boilerplate agreements can be nearly impossible to enforce but may cause serious problems.
A good example is the problem of the clause phrased “no overnights with members of the opposite sex not related by blood or marriage” when you have custody of the kids. Seems like a good idea at face value, but what happens if your best friend from college comes to visit and brings her husband? Technically, you are in violation if he stays in your house. As Helena said about how courts will deal this clause: “When it comes to romantic partner provisions it is hard to determine what the court will do.” Such ambiguity opens up a whole world of trouble. Save yourself any such trouble…get an attorney to review things.
You don’t HAVE to have a separation agreement before obtaining a divorce or equitable distribution of property, although it is recommended that you do. You may file an action of equitable distribution up until the divorce judgement. You are entitled to a percentage of the marital assets, even if you are the one to leave, however, for smaller items, it may be best to sort those out and take them before you leave. Easier to take them right off than try to regain possession when you leave, should things turn sour during the period of separation.