I am not quite sure if your question is asking how to you work with an Interior Designer or if you are wondering how to design your interior by yourself.
Interior Designers are hired to meet you and listen to your wants and needs and then create a design plan for you to approve according to your overall budget. This can include everything within your walls, like, flooring, wall paint, carpets, area rugs, furniture, light fixtures, wall art and lamps, decorative pillows and the final touches with things like nick knacks. OR, you can just hire someone to perhaps re-purpose what you have and help with furniture placement and any other services you may need.
A good interior designer will listen carefully to your likes and dislikes and then come up with a proposal on how to achieve your goals. You can decorate in steps, according to your current and future budget or you can go all out and spend a boat load of money on everything. Your choice. You MUST have a good rapport with your interior designer by having an open line of communication in relaying what you want and the designer understanding your needs and desires.
If you want to decorate yourself, that is a whole other story. I suggest you buy a book that I highly recommend as this book will help you define your style and successfully help you end up with the house of your dreams. Alexandra Stoddard has a book LIVING A BEAUTIFUL LIFE. This book helps you find your own decorating style and allows you to actually achieve it.
The trick to decorating is to do it in layers, beginning with the empty room itself, AFTER you find some inspiration. Paint is the single most least expensive treatment you can use with the most amount of impact. But, where do you begin? You begin with something you love, whether it be a piece of upholstered furniture, a painting, an area rug or a vase. These objects have color inspiration. For instance, if you are decorating a bedroom, buy the bedding you love. From there, pick a color to match the bedding and you cannot fail for a cohesive matching look. If you paint your room a random color first, it is more difficult to match things to it. Unless, of course, if you paint with a neutral color; everything will match a neutral color.
Only buy what you love. Consider second hand items for your home as you can save lots of money and the reason why second hand furniture is still available is because it was well made to begin with. Again, paint can transform anything for little money. Changing drawer pulls and knobs transform an old piece of furniture as well. New furniture is expensive and not built very well unless you spend lots of money. Most things today are built with fake wood, stapled together and the drawers do not even go to the back of the furniture piece...a waste of space.
If you go to Pinterest.com and use keywords of your choice, you can get so many decorating ideas on the cheap.
I hope this has been helpful to you.