You don't unless you entered into it before October 10, 1991. After that date, new Common Law marriages are prohibited in Ohio.
Prior to that you had to meet these requirements:
The six essential elements of a Common Law marriage in Ohio are:
# a mutual agreement of marriage "in praesenti" (presently); # made by persons competent to marry; # followed by cohabitation (including a sexual consummation of the marriage); # a holding out to the public that the parties are actually husband and wife; # a reputation in the community that the parties are husband and wife; and # that the Common Law marriage was entered into before October 10, l991.