Create a motive for your a character that would make them a suspect. Perhaps the character benefited financially from an accident or death within the story, or maybe the victim had put their hot dog stand out of business fifteen years prior with his taco truck. Whatever the reason, make it as believable as the actual motive for the crime in the book. It should not stand out as unlikely, or it will not be successful as a red herring.
Create a character trait that could misdirect your reader. Were there cigarettes found at the scene of the crime? Make them a smoker or a former smoker. Give them a tattoo that is similar to an eye witness account. Even if the character is a red herring, they could prove useful in moving the plot forward and maybe help solve the actual mystery. Maybe the tattoo was a fraternity tattoo given to everyone in a certain fraternity in a certain year, which would move the reader toward solving the mystery while still misdirecting them for a time.
Place the character in the wrong place at the wrong time. Maybe they were hiding out in an alley trying to score on some contraband eight tracks. Or maybe their dog started chasing a rat, and they ended up in a scary alley where someone was just murdered. You can create the appearance of guilt simply by having someone end up in a bad place at an inopportune time.