Alchemy is Etsy's system for requesting custom items. A buyer describes the item they would like and the seller responds with the details of the item they can create. Read about how Alchemy works in our Alchemy Overview.
Being a secretary at heart, I enjoy making orders to specifications. However, I would encourage you to make your custom order request through the "Contact GrandmasGirl" button. Alchemy requests are certainly accepted, but I feel the process is a bit superfluous and accomplished far more easily if I am just asked.
Typically a down payment for custom orders is not required. Particular instances may require one, however, and you will be advised as to the reasons.