Certain models will allow you to direct a customized scene yourself by submitting a script and/or a list of requirements. If you and the model can negotiate an agreement on content and pricing, you can be on your way to owning a one-off fantasy scene created to your own specifications.
The most important thing is to have a contract between you and the model, which outlines precisely what the scene is to entail. The contract should cover what you are to do if the finished product falls short of those criteria, or if the model cannot finish the product in the time agreed upon. Normally, half of the negotiated price is due before filming begins, but models are free to set up their own terms.