How did madam cj walker invent the hot comb?

By putting a metal comb on the stove and making it hot.
Answer #2: Madam Walker did not invent the hot comb. Such heated metal hair care devices existed as early as the 1880s in Europe and in America and were created by numerous inventors when she still was a poor washerwoman working in St. Louis. This myth of Madam Walker inventing the hot comb is very persistent, but it is not accurate. Madam Walker and others used and marketed hot combs and curling irons, but she did not invent them. Pictures of hot combs appear in Bloomingdales and Sears catalogues in the 1890s. Marcel Grateau, a Frenchman who is known for the Marcel Wave, was using metal hair care implements as early as the 1880s and is credited with designing them for use on the hair of wealthy French women.