Your local court will probably have legal agreement templates lying around in case someone would need one. You can also take a look at the free agreement templates that are available and modify it according to your needs and the needs of your partner.
What you will typically find are free agreement forms. Make sure you fill out the form and make all the necessary edits to match it with separation situation that you and you are going through and have come to an agreement with. You can add a few personal notes or paragraphs should it be deemed necessary.