1) Voters must approve a referendum to convene a con-con.
2) Voters elect delegates to the con-con from the same districts in which they elect members of the General Assembly. (Historically, only a small percentage of those delegates are incumbent legislators because voters recognize such incumbents have a conflict of interest in serving on a con-con.)
3) The con-con delegates meet in Annapolis and propose one or more constitutional amendments for the voters to ratify.
4) The voters must ratify any proposed amendments before they can become law. The ratification process for amendments proposed by a con-con is identical to that for amendments proposed by the General Assembly.
In summary, voters have three votes in the process: 1) to convene the con-con, 2) to elect delegates to the con-con, and 3) to ratify any proposals the con-con delegates seek to put on the ballot. An additional check is that no proposition can be placed on the ballot without approval from a majority of the elected delegates.
For a discussion about common con-con myths, see Snider and Tarr's "A Historic Year for State Con-Cons" in the Featured Articles box below.
Seth Johnson on why you need to sign the petition:
"If you are in Maryland, you need to endorse this petition -- regardless of what you think about having a constitutional convention. That has already been voted on. If you believe in government of, by, and for the people, you need to claim your authority and standing as the source of your government's powers and limits. If you have an interest in reclaiming recourse over your government, demanding recognition of the nature of your government by endorsing this petition is a very direct means by which you can bring that cause to bear."
That all Government of right originates from the People...; and they have, at all times, the inalienable right to alter, reform or abolish their Form of Government in such manner as they may deem expedient.
It shall be the duty of the General Assembly to provide by Law for taking, at the general election to be held in the year nineteen hundred and seventy, and every twenty years thereafter, the sense of the People in regard to calling a Convention for altering this Constitution; and if a majority of voters at such election or elections shall vote for a Convention, the General Assembly, at its next session, shall provide by Law for the assembling of such convention, and for the election of Delegates thereto. Each County, and Legislative District of the City of Baltimore, shall have in such Convention a number of Delegates equal to its representation in both Houses at the time at which the Convention is called. But any Constitution, or change, or amendment of the existing Constitution, which may be adopted by such Convention, shall be submitted to the voters of this State, and shall have no effect unless the same shall have been adopted by a majority of the voters voting thereon (amended by Chapter 99, Acts of 1956, ratified Nov. 6, 1956).
The Maryland State Constitutional Convention Clearinghouse 