IF this is something you want to do, work up a detailed estimate, and don't scrimp on your estimations of the cost of things (i.e., don't figure the fabric will be on sale) or the time it will take to do it. Decide on an hourly amount, above minimum wage, and present her with the estimate. I'm betting she will not take you up on it. If she does, both of you sign the contract, which should specify terms and the amount of deposit.