hang out one's shingle - Phrases/Idioms
The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Open an office, especially a professional practice, as in Bill's renting that office and hanging out his shingle next month. This American colloquialism dates from the first half of the 1800s, when at first lawyers, and later also doctors and business concerns, used shingles for signboards.