Can we write the premarital agreement ourselves, without a family law attorney?

Legally, you can write the prenuptial agreement yourself, but this is not a good idea. A lawyer who specializes in family law is in the best position to advise you as to the advantages and pitfalls of certain provisions, how they can best be drafted to take into account your particular needs - and whether a premarital agreement is even good for your situation. Consult with a family law attorney in your  area.