If you are working with a wedding planner, find one that has a niche for decor – as you will give them a list of colors and themes and any specific wedding decorations you want and they will run with it (hopefully). If you're working by yourself, looking at examples and try to picture in your head exactly what you want. Write down specifics and organize your thoughts in an outline on paper before executing it.
It is your big day and everyone has a different idea of the perfect wedding decorations for them. Find a theme and a few main colors to stick with – if you use too many colors it will look too buys. One good rule of thumb is picking one color and using lighter/darker shades along with neutrals.