If you need a ring soon, then a custom designed ring will not be a solution for you. Give enough time for your jeweler to order the stone, set design and manufacture the ring. This can vary from jewelry store to jewelry store.
* Keep in mind that a custom jeweler you will not be able to read your mind. Taking pictures together from all the things you want in a custom engagement ring is a good place to start. High-quality custom jeweler will be able to provide a computer image or picture what your custom engagement ring will look like before starting the job. This is the right time for a little misunderstanding to be ironed out.