Everyone's interests are best served by writing your agreements on a piece of paper with your case number on it, signing it and filing it with the court. That way you have an enforceable order. You can go to the Colorado State Court website and download the appropriate form if you prefer. The one issue with your daughter living with her dad for a year is that he will have the right to file to modify the parenting plan after she has been with him for 182 days. Given that she will be 17 at that time, if she does not want to be with you or her dad, I would hope you both will figure out how she can best spend the time between then and her 18 birthday when she can chose where to live and how to live her life. Congratulations on trying to work this out yourselves.
Assuming that you already have a custody order AND further, in order to protect yourself, you need to go to the same court that issued your existing custody order to modify it. Only the court can modify it (with authority). I suggest that you and he file a joint motion. The "joint" motion will imply that you and he both agree to the contents. Keep in mind, if you and he do this extra-judicailly, you will not be able to have the court enforce anything that is not a part of the record. If something happens that is not a part of the agreement, then you always have the option to go back to court to enforce or modify the agreement. The decision is yours but if you already have trust issues with your ex, and you are fearful that he may not uphold his part of the agreement, then you need to go to court to modify it.
<<DISCLAIMER>> Information provided in this response is intended to be informational or educational only. It in no way establishes an attorney-client relationship. Because every case is factually (and in some instances jurisdictionally) dependent, it is not possible to accurately answer each question posed. If you have sincere legal concerns, it is highly recommended that you seek legal counsel IN YOUR AREA. A response is not intended to create, nor does it create, a continuing duty to respond. Also, Ms. Eskinde is licensed in Louisiana, so if you require immediate assistance, then you need to contact someone who is in your state, if outside of Louisiana.