Firstly, choose a maker whose work you like. Then form an outline idea in your mind of the project, its use and location, and approximate size. A rough sketch is sometimes useful as can be a photograph of where the piece is to sit.
Have a reasonable idea of your budget; this is an important step towards creating a good and open working relationship with your chosen designer. Ask if any other fees are likely to be encountered, and if payments are to be made in instalments (a possibility with large scale commissions).
Most members welcome commissions and are happy to work with you to design and create a piece that is unique to you which will give pleasure for many years to come.