Choose a style
Firstly, you'll need to decide on a basic style. It's helpful to visit museums and browse design books, but the best inspiration will come from your home and the furniture you already have. For instance, you probably won't want to commission something in an Art Deco style if the rest of the furniture is Georgian.
The nitty gritty
Once you've decided on a style, it's worth making a few notes about what you want from the design. If it's a window seat, will you want it to have hidden storage underneath? If it's a display cabinet, will you want open shelving or glass doors? You can discuss these ideas with your cabinet-maker, who will no doubt also have some of their own suggestions.