What it will cost: Interior designers have various fee structures. They might charge an hourly rate (anywhere from $50 to $500); a flat fee, which could be as low as several thousand dollars and as high as five figures; or a percentage of total project costs. You may also be asked to pay a retainer before work begins.

Some designers take an approach known as cost-plus — they purchase materials, furnishings and more at a discount, then charge you the retail price, keeping the difference as their fee. If yours uses this method, be sure the cost discrepancies are transparent so that you'll know exactly what you're paying for services.
If you're on a tight budget, don't assume that hiring a designer is beyond reach. Many will be happy to arrange a few hours of consulting or will help you source furnishings and decorative accents.

Where to find one: The directory of interior designers on Houzz is a good starting place, where you can find designers in your area and beyond and view their portfolios. Your local chapter of the American Society of Interior Designers (ASID) is also a good source.

If you spot a room you love when browsing Houzz photos and ideabooks, take note of the designer's name. (You'll see a link to professionals' profiles in the lower-right corner of their photos.) Some designers will work with clients who are based in other cities, though travel time and expenses can add up.

Friends with fabulous houses are another likely source. You can also visit show houses and home tours to see which spaces strike your fancy.

Be sure the designer is a good match for your style. No two clients are alike, and good interior designers are nimble enough to hop from urban pied-à-terre to rustic farmhouse to beachside getaway without missing a beat. However, almost all of them have a fundamental aesthetic that remains consistent throughout their work. For example, if you want pure whites and pale neutrals, don't choose a designer whose signature is bold color palettes. When interviewing designers, look for parallels between their previous work and the design you want.

Collect samples. Even if you have trouble articulating your desired look, pictures of rooms you love can instantly give the designer a sense of what you crave. He or she will ask you about specific points of the design that resonate with you and use those as guidelines. Fabric swatches, paint chips, furniture catalogs and your own Houzz ideabooks are other good sources for showing items you like. On the flip side, pull examples of colors, motifs and furniture styles that turn you off, which can be equally helpful.

Find your design style

Decide in advance which pieces must stay.Not willing to get rid of your Biedermeier sideboard or your majolica collection? The process will go more smoothly if you share that information with your designer during the initial site visit and consultation. That way, he or she can plan around the items that you don't want to give up.

Involve the designer as early as possible in the building process. If you're remodeling or building from scratch, include the designer in the planning stages with your architect and contractor. This way, the pros involved will all be on the same page and can iron out any potential discrepancies — particularly those that involve the bones of a home, such as doorways, ceiling beams or interior columns. It's one thing to reorient a window on paper; it's another entirely to move it after installation.

Find thousands of photos of every style of home

Keep an open mind. It's a rare client who loves 100 percent of a designer's suggestions right off the bat. Your designer might recommend a piece of furniture or a wallpaper pattern that you're iffy about, but don't say no without giving the idea some time to sink in. Chances are that when you ask your designer why he or she chose it, and when you take a little time to live with it, you'll appreciate the reason it works.

Look toward refreshing down the road. Even the best design doesn't stay current forever. Ask your designer if tune-up visits in the future are an option, whether they involve simply swapping out a few accessories, reupholstering furniture or choosing new paint colors. 

Reference  : http://www.houzz.com/ideabooks/3293842/list/how-to-work-with-an-interior-designer