Source: http://www.politicalcartoons.com/cartoon/ad6484c6-1959-4321-8701-913f258c93ef.html
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."