It does not make any difference which you do first. However, the strong recommendation is that, six to twelve months before your wedding date, you contact a Catholic Church where you attend mass OR near where you live. They will set up a meeting with you and a priest or deacon from the parish. One of the documents you will be asked for at that time is the marriage preparation completion certificate. If you have it already, you can provide it at that time. If not, you can submit it to the parish whenever you take the program.