Don't try to do this yourself. Not sure about Missouri law because I'm licensed to practice only in Wisconsin, however, if you have not already done so you will need to have your Family Court Judge sign a Qualified Domestic Relations Order which divides your ex-husbands pension pursuant to the order in the Judgment of Divorce and will then be filed with GM's Pension Administrator. Please contact a local family law attorney in your area to assist you in this regard. S/He may even contract this out to a third party specialist to prepare this Order since the Pension Administrators are very picky that these need to be prepared with specificity.
The process is relatively easy. It requires a call to the employer, G.M., and asking them to speak to the plan administrator. Then, once you have him or her on the line, ask them for a sample Qualified Domestic Relations Order, QDRO, that they approve of.
Once you have that, simply complete the QDRO and file with the court. In order to file with the court, sometimes you can simply walk in (with a copy of the Divorce Judgment) and have the QDRO entered and others you must motion in advance and give the other side notice of your motion.
In court, the judge will examine the QDRO and if it comports with the Divorce Judgment, she/he will enter the QDRO.
Final step, once entered, serve the QDRO on the plan administrator. Usually certified mail is the best. That is all there is to it.
The earlier answers are on point. The key is getting a court order that both the employer (GM) and the court find acceptable. I would definately suggest seeking out an attorney to assist you with this process.