A new General Synod is elected every five years and meets two or three times a year. It comprises three Houses: Bishops, Clergy and Laity. The number of members given below is for the 2005-2010 Synod.
Members vote according to their own conscience; nobody can instruct them how to vote.
Members vote as individuals; there is no voting by diocese as in the USA. The results of votes are decided by counting the numbers of members voting for and against a motion. In most cases the count is of the whole Synod and a simple majority is required for a motion to be passed. Sometimes each House votes separately (and then each House must vote in favour) and in some of these cases a two-thirds majority is required in each House.
A simplified account of how Synod is elected follows below the fold.
House of Bishops
The two archbishops and all the other diocesan bishops (and the Bishop of Dover, who is effectively the diocesan bishop for Canterbury) are members. In addition the suffragan bishops elect seven of their number to the House.
House of Clergy
Each diocese elects a number of clergy (182 in 2005); larger dioceses elect more than smaller dioceses. The electors are basically the licensed clergy plus a small number of retired clergy. Those eligible to stand for election are the electors plus clergy with permission to officiate (mainly retired clergy).
House of Laity
Each diocese elects a number of laity (193 in 2005); larger dioceses elect more than smaller dioceses. The electors are the lay members of deanery synods. Candidates must be over 18, actual communicants, and on an electoral roll (or the equivalent roll in a cathedral).
These include seven members from the armed services, five cathedral deans, four members of religious communities, six university clergy, and a few ex officio members (eg members of the Archbishops’ Council). This gives a total membership of 466. In addition up to ten people can be co-opted to membership.
There is a lot of official information about the 2005 elections here, particularly the Notes for the Guidance of Dioceses.
Candidates for election require a proposer and seconder who are qualified electors.
Elections are by the single transferable vote. This article includes a good explanation of how the STV system works.