Designers work in many different ways. Typically, only the ones with big namesinsist on doing the entire house. Some charge an hourly fee, some just markup what they purchase (usually from to-the-trade-only sources), some do a combination of both.

A designer will help you with all your choices and will have workrooms they use for curtains, upholstery, and will often also have paper hangers and painters they recommend as well.

Just be sure that your designer’s vision in the same as yours. Many designers work for the same clients for years, as the client has the money to do more rooms.

I've used designers in the past, and recently hired a designer to help with some of the interior design decisions on a house we will be building. Our architect gave us a few recommendations, and I looked at portfolios here on Houzz as well. We ended up interviewing four designers and chose the person who we felt most comfortable with. She's fun, easy-going and is very conscious of our budget and how to get the most "bang for the buck." In fact, we met with her recently to sort through which of our existing furniture will be repurposed for the new house (some "as is" and some reupholstered/refinished). She will be helping us with color schemes, selecting new furniture, lighting, and some accessories.

Ask around for recommendations, interview and look at portfolios, and go with someone you think would be enjoyable to work with. The process should be collaborative; don't let your designer talk you into anything that you aren't comfortable with. On the other hand, be open to ideas he/she brings to the table that you hadn't considered; the creativity and experience a good designer brings to the project will result in a home you love to live in!

I’ve always gotten my decorators through friends recommendations. Each of the 3 I’ve worked with began with a relatively smaller project but since I liked them and their work, the assignments expanded over time. Each of them used a combination of by the hour and a percentage of items I purchased (since I also got the benefit of their trade discount, the costs were not exorbitant). They helped with everything — color, furnishings, fixtures, lights, floors, kitchen design, workmen, etc., etc. They saved me a lot of money over the years, both from not buying stuff that didn’t work, as well as providing highly qualified workers such as painters, tilers, upholsterers, Electricians, contractors, etc. And, when on the rare occasions where there was a problem with their sub or provider, they dealt with getting things corrected at no cost to me.