If you are lucky enough to have a stand-out engineering team, and know what you need, you can simply email the specs directly to the manufacturer. Even if the job is custom, you may have had a similar one created previously. Perhaps your team just understands custom cable manufacturing and is able to create the proper specs on their own, without guidance from the supplier. Before your final order is placed, they will still review the request with you, but emailing the specs is for the customer that has a good understanding of what they need going into the ordering process.
Discuss options over the phone
If your team is mostly sure of what is necessary for the job, but still has a few questions before the final order can be placed, then a call to a manufacturer is a good idea. The professionals there will be able to talk with you to narrow down your options and help you make a decision. They will likely also be able to make suggestions about other products that could be better than what you had in mind, in order to improve cost or function. Since you already have a good understanding of what is most likely needed, the process should be smooth and not take much time.
Share a mock-up, sketch, or model
When words simply fail and you aren’t able to describe what you need in written or spoken form, then it is time to turn to images. Use what you have, or what you are good at doing. A sketch on paper is a great place to start. Often, the custom cable manufacturing team has come across similar jobs before and will be able to create a few options for you. A model, is also helpful. It could be as complex or crude as you are able to make. A virtual model, created with design software is always helpful. Obviously, it is important to properly illustrate key elements, but the mock-up doesn’t have to be a perfect working replica.