Legal Question in Family Law in California

You can, but without knowledge of all the ins and outs of California Family Law there is a high likelihood of you putting something in or leaving something out that will invalidate it or otherwise trip you up if you ever want to or have to enforce it. As for filing it, you don't. It is just like any other contract. The parties should each keep a signed copy. It would only be filed if it is to be introduced in court as evidence.