Arrange interviews with a few potential Interior Designers in your area. Ask about their prior projects, education, experience, and client references. Upon your request they should also be able to provide you with a portfolio of their work and any professional affiliations or credentials. Interviewing different designers will enable you to become familiar with differences in personality, style preferences, and business practices. Remember that you will be working closely with this person, so make sure you feel comfortable interacting and expressing your ideas with them.
Once you have narrowed your search, inquire further about the designer’s fee structure.
Is there a contract? Discuss your specific needs to make sure the designer’s fee schedule/contract will work for you. For example if you only need some advice on paint colors and furniture placement, a flat or hourly fee would be most common. However, if you project is more involved, there will likely be a combination of fees for renderings, research & development, and mark-ups on purchased items etc. Speak openly with the designer about your budget, and let them know if the number is firm or flexible.
Lastly, when you do hire a designer remember to be upfront and honest with them at all times. Speaking up in the beginning allows them to better understand your vision for the space, and design accordingly. When designer and client are on the same page things flow smoothly and the space you envisioned can be realized.