I need a private agreement/basic contract written.

I am in the middle of moving house and have come to an agreement with the people who are about to buy my flat.
We wish this to be a private agreement, separate to both party's lawyers. The basic agreement is that I will pay for the flat's share of a bill that may occur due to building works that are happening at the moment.
I have made a bullet point list of what needs to be included but we need help to convert it from a list to a formal agreement, with spaces for both of us to sign. Could you help?